Remove obsolete -mno-cygwin flag

GCC removed the `-mno-cygwin` option in the 4.x series, so any GCC
recent enough to build Ruby rejects it and the dllwrap path for
`--disable-shared` builds fails with it.

cygwin/GNUmakefile.in

@@ -16,7 +16,6 @@ ifeq ($(target_os),cygwin)
   DLL_BASE_NAME := $(LIBRUBY_SO:.dll=)
 else
   DLL_BASE_NAME := $(RUBY_SO_NAME)
-  DLLWRAP += -mno-cygwin
   VPATH := $(VPATH):$(srcdir)/win32
   ifneq ($(filter -flto%,$(LDFLAGS)),)
     miniruby$(EXEEXT): XLDFLAGS += -Wno-maybe-uninitialized