Consolidate the glob case logic

Looking at the code as a whole, we ended up with a little to much "buttons". It turns out that doing case insensitive matching (lower both pattern and strings to match) performs just fine. Or at least, it
gives the penalty to the people who uses mixed case filenames.

```
GetGlob/Default_cache-10                          10.6ns ± 2%    10.6ns ± 1%   ~     (p=0.657 n=4+4)
GetGlob/Filenames_cache,_lowercase_searchs-10     10.6ns ± 2%    10.6ns ± 0%   ~     (p=1.000 n=4+4)
GetGlob/Filenames_cache,_mixed_case_searchs-10    29.7ns ± 1%    29.6ns ± 1%   ~     (p=0.886 n=4+4)
GetGlob/GetGlob-10                                13.7ns ± 1%    13.7ns ± 0%   ~     (p=0.429 n=4+4)

name                                            old alloc/op   new alloc/op   delta
GetGlob/Default_cache-10                           0.00B          0.00B        ~     (all equal)
GetGlob/Filenames_cache,_lowercase_searchs-10      0.00B          0.00B        ~     (all equal)
GetGlob/Filenames_cache,_mixed_case_searchs-10     5.00B ± 0%     5.00B ± 0%   ~     (all equal)
GetGlob/GetGlob-10                                 0.00B          0.00B        ~     (all equal)

name                                            old allocs/op  new allocs/op  delta
GetGlob/Default_cache-10                            0.00           0.00        ~     (all equal)
GetGlob/Filenames_cache,_lowercase_searchs-10       0.00           0.00        ~     (all equal)
GetGlob/Filenames_cache,_mixed_case_searchs-10      1.00 ± 0%      1.00 ± 0%   ~     (all equal)
GetGlob/GetGlob-10
```

1 files changed, 4 insertions, 38 deletions

diff --git a/hugofs/glob/glob.go b/hugofs/glob/glob.go
index 87619802e..841824bcc 100644
--- a/hugofs/glob/glob.go
+++ b/hugofs/glob/glob.go
@@ -33,11 +33,6 @@ var (
 		isWindows: isWindows,
 		cache:     make(map[string]globErr),
 	}
-
-	filenamesGlobCache = &globCache{
-		isWindows: isWindows,
-		cache:     make(map[string]globErr),
-	}
 )
 
 type globErr struct {
@@ -86,10 +81,6 @@ func (gc *globCache) GetGlob(pattern string) (glob.Glob, error) {
 }
 
 type globDecorator struct {
-	// Whether both pattern and the strings to match will be matched
-	// by their original case.
-	isCaseSensitive bool
-
 	// On Windows we may get filenames with Windows slashes to match,
 	// which wee need to normalize.
 	isWindows bool
@@ -101,14 +92,12 @@ func (g globDecorator) Match(s string) bool {
 	if g.isWindows {
 		s = filepath.ToSlash(s)
 	}
-	if !g.isCaseSensitive {
-		s = strings.ToLower(s)
-	}
+	s = strings.ToLower(s)
 	return g.g.Match(s)
 }
 
 type globDecoratorDouble struct {
-	lowerCase   glob.Glob
+	lowerCase    glob.Glob
 	originalCase glob.Glob
 }
 
@@ -120,34 +109,11 @@ func GetGlob(pattern string) (glob.Glob, error) {
 	return defaultGlobCache.GetGlob(pattern)
 }
 
-func GetFilenamesGlob(pattern string) (glob.Glob, error) {
-	lowered := strings.ToLower(pattern)
-	hasUpperCase := pattern != lowered
-	gLowered, err := filenamesGlobCache.GetGlob(lowered)
-	if err != nil {
-		return nil, err
-	}
-
-	if !hasUpperCase {
-		return gLowered, nil
-	}
-
-	gSensitive, err := filenamesGlobCache.GetGlob(pattern)
-	if err != nil {
-		return nil, err
-	}
-	return globDecoratorDouble{
-		lowerCase:   gLowered,
-		originalCase: gSensitive,
-	}, nil
-
-}
-
 func NormalizePath(p string) string {
-	return strings.Trim(path.Clean(filepath.ToSlash(strings.ToLower(p))), "/.")
+	return strings.ToLower(NormalizePathNoLower(p))
 }
 
-func NormalizePathCaseSensitive(p string) string {
+func NormalizePathNoLower(p string) string {
 	return strings.Trim(path.Clean(filepath.ToSlash(p)), "/.")
 }