Remove long-gone cxxmain.c from makefiles

I discovered we were attempting to delete some no-longer generated
files.

	gcc/
	* Makefile.in (distclean): Remove long gone cxxmain.c
	gcc/cp/
	* Make-lang.in (c++.disclean): Likewise.

gcc/Makefile.in

@@ -3501,7 +3501,6 @@ distclean: clean lang.distclean
 	-cd testsuite && rm -f x *.x *.x? *.exe *.rpo *.o *.s *.S *.c
 	-cd testsuite && rm -f *.out *.gcov *$(coverageexts)
 	-rm -rf ${QMTEST_DIR} stamp-qmtest
-	-rm -f cxxmain.c
 	-rm -f .gdbinit configargs.h
 	-rm -f gcov.pod
 # Delete po/*.gmo only if we are not building in the source directory.

gcc/cp/Make-lang.in

@@ -287,7 +287,6 @@ c++.mostlyclean:
 c++.clean:
 c++.distclean:
 	-rm -f cp/config.status cp/Makefile
-	-rm -f cxxmain.c
 c++.maintainer-clean:
 #
 # Stage hooks: