git-remote-mediawiki: do not remove installed files in "clean" target

Running "make clean" after a successful "make install" should not
result in a broken mediawiki remote helper.

Reported-by: Thorsten Glaser <t.glaser@tarent.de>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Acked-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/mw-to-git/Makefile

@@ -31,7 +31,6 @@ install: install_pm
 clean:
 	$(MAKE) -C $(GIT_ROOT_DIR) SCRIPT_PERL=$(SCRIPT_PERL_FULL) \
                 clean-perl-script
-	rm $(INSTLIBDIR)/$(GIT_MEDIAWIKI_PM)
 
 perlcritic:
 	perlcritic -2 *.perl