Makefile: remove *.spec from clean target

Support for generating an rpm was dropped in ab214331cf ("Makefile: stop
pretending to support rpmbuild", 2016-04-04).  We don't generate any
*.spec files so there is no need to clean them up.

Signed-off-by: Todd Zullinger <tmz@pobox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -2456,7 +2456,7 @@ clean: profile-clean coverage-clean
 	$(RM) $(TEST_PROGRAMS) $(NO_INSTALL)
 	$(RM) -r bin-wrappers $(dep_dirs)
 	$(RM) -r po/build/
-	$(RM) *.spec *.pyc *.pyo */*.pyc */*.pyo common-cmds.h $(ETAGS_TARGET) tags cscope*
+	$(RM) *.pyc *.pyo */*.pyc */*.pyo common-cmds.h $(ETAGS_TARGET) tags cscope*
 	$(RM) -r $(GIT_TARNAME) .doc-tmp-dir
 	$(RM) $(GIT_TARNAME).tar.gz git-core_$(GIT_VERSION)-*.tar.gz
 	$(RM) $(htmldocs).tar.gz $(manpages).tar.gz