NativeMethods: drop unnecessary libgit2 refcount
NativeMethods: drop unnecessary libgit2 refcount

Remove the refcounting of libgit2 usage leftover from when
`SafeHandleBase` ensured that all handles were finished before calling
`git_libgit2_shutdown`.

libgit2 itself does this refcounting now, we no longer need to.  We only
need to call `git_libgit2_init` before the first call to libgit2 and
`git_libgit2_shutdown` at the end.

PKRoma/libgit2sharp,Zoxive/libgit2sharp,libgit2/libgit2sharp,Zoxive/libgit2sharp