Remove extern from function declared in headers. (#13790)

By default functions are declared as extern in C/C++ headers. The goal
of this PR is to reduce the wall of text that many headers have and,
more importantly, to make the declaration of extern'd variables - of
which we have many dispersed in various places - easily and quickly
identifiable.

Automatically generated with:

    $ git grep -l '^extern.*(' '**.h' | \
            grep -v libjudy | \
            grep -v 'sqlite3.h' | \
            xargs sed -i -e 's/extern \(.*(.*$\)/\1/'

This is a NFC.

3 files changed, 6 insertions, 6 deletions

diff --git a/exporting/exporting_engine.h b/exporting/exporting_engine.h
index 2141caa41a..5f961c303a 100644
--- a/exporting/exporting_engine.h
+++ b/exporting/exporting_engine.h
@@ -271,7 +271,7 @@ size_t exporting_name_copy(char *dst, const char *src, size_t max_len);
 int rrdhost_is_exportable(struct instance *instance, RRDHOST *host);
 int rrdset_is_exportable(struct instance *instance, RRDSET *st);
 
-extern EXPORTING_OPTIONS exporting_parse_data_source(const char *source, EXPORTING_OPTIONS exporting_options);
+EXPORTING_OPTIONS exporting_parse_data_source(const char *source, EXPORTING_OPTIONS exporting_options);
 
 NETDATA_DOUBLE
 exporting_calculate_value_from_stored_data(
@@ -300,7 +300,7 @@ void create_main_rusage_chart(RRDSET **st_rusage, RRDDIM **rd_user, RRDDIM **rd_
 void send_main_rusage(RRDSET *st_rusage, RRDDIM *rd_user, RRDDIM *rd_system);
 void send_internal_metrics(struct instance *instance);
 
-extern void clean_instance(struct instance *ptr);
+void clean_instance(struct instance *ptr);
 void simple_connector_cleanup(struct instance *instance);
 
 static inline void disable_instance(struct instance *instance)
diff --git a/exporting/prometheus/prometheus.h b/exporting/prometheus/prometheus.h
index 4b8860ded8..e80b682ae5 100644
--- a/exporting/prometheus/prometheus.h
+++ b/exporting/prometheus/prometheus.h
@@ -22,10 +22,10 @@ typedef enum prometheus_output_flags {
 	PROMETHEUS_OUTPUT_HIDEUNITS  = (1 << 6)
 } PROMETHEUS_OUTPUT_OPTIONS;
 
-extern void rrd_stats_api_v1_charts_allmetrics_prometheus_single_host(
+void rrd_stats_api_v1_charts_allmetrics_prometheus_single_host(
     RRDHOST *host, const char *filter_string, BUFFER *wb, const char *server, const char *prefix,
     EXPORTING_OPTIONS exporting_options, PROMETHEUS_OUTPUT_OPTIONS output_options);
-extern void rrd_stats_api_v1_charts_allmetrics_prometheus_all_hosts(
+void rrd_stats_api_v1_charts_allmetrics_prometheus_all_hosts(
     RRDHOST *host, const char *filter_string, BUFFER *wb, const char *server, const char *prefix,
     EXPORTING_OPTIONS exporting_options, PROMETHEUS_OUTPUT_OPTIONS output_options);
 
@@ -36,6 +36,6 @@ char *prometheus_units_copy(char *d, const char *s, size_t usable, int showoldun
 
 void format_host_labels_prometheus(struct instance *instance, RRDHOST *host);
 
-extern void prometheus_clean_server_root();
+void prometheus_clean_server_root();
 
 #endif //NETDATA_EXPORTING_PROMETHEUS_H
diff --git a/exporting/prometheus/remote_write/remote_write.h b/exporting/prometheus/remote_write/remote_write.h
index 4740772d06..d4e86494b4 100644
--- a/exporting/prometheus/remote_write/remote_write.h
+++ b/exporting/prometheus/remote_write/remote_write.h
@@ -18,7 +18,7 @@ struct prometheus_remote_write_variables_callback_options {
 };
 
 int init_prometheus_remote_write_instance(struct instance *instance);
-extern void clean_prometheus_remote_write(struct instance *instance);
+void clean_prometheus_remote_write(struct instance *instance);
 
 int format_host_prometheus_remote_write(struct instance *instance, RRDHOST *host);
 int format_chart_prometheus_remote_write(struct instance *instance, RRDSET *st);