Removed obsolete forwarding DLL recipe

It has never been created by default, and no reason to keep binary
compatibility against extension libraries for ruby 1.8.

1. `LIBRUBY_SO` on Cygwin was named originally.

   - 94089e82a12:: `$(RUBY_INSTALL_NAME)-cygwin.dll`
   - f0ba57341ac:: `$(RUBY_INSTALL_NAME).cygwin.dll`
   - c74a3b21f59:: `cygwin-$(RUBY_INSTALL_NAME)-$(MAJOR).$(MINOR).dll`

2. And then renamed to be net distribution compliant.

   - 6f059f33ef1:: `cyg$(RUBY_SO_NAME)$(MAJOR)$(MINOR).dll`

3. Next at 430eabfe13, forwarding DLL was added for binary
   compatibility against extension libraries.

cygwin/GNUmakefile.in

@@ -97,12 +97,6 @@ $(RUBYDEF): $(LIBRUBY_A) $(PREP) $(RBCONFIG)
 clean-local::
 	@$(RM) $(RUBYDEF)
 
-ifeq (@target_os@,cygwin)
-cygwin-$(RUBY_INSTALL_NAME)$(MAJOR)$(MINOR)0.dll: $(LIBRUBY_A) $(RUBYDEF)
-	$(ECHO) generating $@
-	$(Q) @DLLWRAP@ -s --def=$(RUBYDEF) -o $@
-endif
-
 clean-local::
 	@$(RM) $(RUBY_EXP) $(RCFILES:.rc=.res.@OBJEXT@)
 	@$(RM) $(RCFILES)