Cygwin: Drop STDINCFLAGS overrides

This used to turn off -nostdinc on a per-file basis, but has no effect
since 4c36016b57.

winsup/cygwin/Makefile.in

@@ -583,10 +583,6 @@ exec_CFLAGS:=-fno-builtin-execve
 fhandler_proc_CFLAGS+=-DUSERNAME="\"$(USER)\"" -DHOSTNAME="\"$(HOSTNAME)\""
 fhandler_proc_CFLAGS+=-DGCC_VERSION="\"`$(CC) -v 2>&1 | tail -n 1`\""
 
-_cygwin_crt0_common_STDINCFLAGS:=yes
-libstdcxx_wrapper_STDINCFLAGS:=yes
-cxx_STDINCFLAGS:=yes
-
 .PHONY: all force dll_ofiles install all_target install_target all_host \
 	install_host install install-libs install-headers \
 	clean distclean realclean maintainer-clean