mingw: remove unnecessary definition

For some reason, the definition of the MINGW version of
`mark_as_git_dir()` slipped into this developer's patch series to
support building Git for Windows.

As the `mark_as_git_dir()` function is not needed at all anymore (it was
used originally to support the core.hideDotFiles = gitDirOnly setting,
but we now use a different method to support that case), let's just
remove it.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/mingw.h

@@ -416,9 +416,6 @@ int mingw_offset_1st_component(const char *path);
 void mingw_open_html(const char *path);
 #define open_html mingw_open_html
 
-void mingw_mark_as_git_dir(const char *dir);
-#define mark_as_git_dir mingw_mark_as_git_dir
-
 /**
  * Converts UTF-8 encoded string to UTF-16LE.
  *