remove mutex.h from the distribution

This header is an internal header and should not be exposed to the user.
This fully internalises the concept of thread safety to the build.

src/CMakeLists.txt

@@ -84,7 +84,6 @@ install(FILES
   include/inlines.h
   include/iterator.h
   include/map.h
-  include/mutex.h
   include/node.h
   include/parser.h
   include/plugin.h

src/include/module.modulemap

@@ -11,7 +11,6 @@ module cmark_gfm {
     header "inlines.h"
     header "iterator.h"
     header "map.h"
-    header "mutex.h"
     header "node.h"
     header "parser.h"
     header "plugin.h"