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>