When checking whether a rule negates another rule, we were checking
whether a rule had the `GIT_ATTR_FNMATCH_LEADINGDIR` flag set and, if
so, added a "/*" to its end before passing it to `fnmatch`. Our code now
sets `GIT_ATTR_FNMATCH_NOLEADINGDIR`, thus the `LEADINGDIR` flag shall
never be set. Furthermore, due to the `NOLEADINGDIR` flag, trailing
globs do not get consumed by our ignore parser anymore.

Clean up code by just dropping this now useless logic.