Clean up MinGW def file generation

There were some leftovers from ancient ad-hoc ways to build on
Windows, prior to the standardization on MSVC and MinGW.  We don't
need to build a lib$(NAME)ddll.def (debug build, as opposed to
lib$(NAME)dll.def) for MinGW, since nothing uses that.  We also don't
need to build the regular .def file during distprep, since the MinGW
build environment is perfectly capable of creating that normally at
build time.

Discussion: https://www.postgresql.org/message-id/flat/0f9db9f8-47b8-a48b-6ccc-15b22b412316%402ndquadrant.com

src/interfaces/ecpg/compatlib/Makefile

@@ -54,4 +54,4 @@ uninstall: uninstall-lib
 clean distclean: clean-lib
 	rm -f $(OBJS)
 
-maintainer-clean: distclean maintainer-clean-lib
+maintainer-clean: distclean

src/interfaces/ecpg/ecpglib/Makefile

@@ -57,4 +57,4 @@ uninstall: uninstall-lib
 clean distclean: clean-lib
 	rm -f $(OBJS)
 
-maintainer-clean: distclean maintainer-clean-lib
+maintainer-clean: distclean

src/interfaces/ecpg/pgtypeslib/Makefile

@@ -44,4 +44,4 @@ uninstall: uninstall-lib
 clean distclean: clean-lib
 	rm -f $(OBJS)
 
-maintainer-clean: distclean maintainer-clean-lib
+maintainer-clean: distclean

src/interfaces/libpq/Makefile

@@ -139,6 +139,6 @@ clean distclean: clean-lib
 # Remove files we (may have) symlinked in from other places
 	rm -f encnames.c wchar.c
 
-maintainer-clean: distclean maintainer-clean-lib
+maintainer-clean: distclean
 	$(MAKE) -C test $@
 	rm -f libpq-dist.rc