maint: remove obsolete/broken maintainer targets

maint: remove obsolete/broken maintainer targets

* Makefile.am (git-diff): Remove.  First, we don't distribute diffs
between an automake version and the next one anyway, so this target
is pointless.   Moreover, its recipe has been broken for quite a
long time now, always generating an empty diff due to the spurious
'$(PACKAGE)' argument that was passed to the "git diff" invocation.
(path-check): Remove.  The 'pathchk' program running in this check
complains about any file with name longer than 14 characters, which
is a ridiculously low limit for today standards.  Also, we already
had several test scripts (no less than 195!) that were exceeding
that limit, and nobody ever complained (not even on MinGW/MSYS nor
Cygwin).