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.

1 files changed, 1 insertions, 1 deletions

diff --git a/ml/ml.h b/ml/ml.h
index 4b3e0754ab..773714a3fb 100644
--- a/ml/ml.h
+++ b/ml/ml.h
@@ -12,7 +12,7 @@ extern "C" {
 
 // This is a DBEngine function redeclared here so that we can free
 // the anomaly rate dimension, whenever its backing dimension is freed.
-extern void rrddim_free(RRDSET *st, RRDDIM *rd);
+void rrddim_free(RRDSET *st, RRDDIM *rd);
 
 typedef void* ml_host_t;
 typedef void* ml_dimension_t;