pgxs.mk: remove unreachable rule for deleting regress.def.

We never create regress.def, and if we did this code would fail to
delete it, because "win" is not the correct PORTNAME for Windows.

This thinko seems to have originated in commit 7a6b562fd from 1999,
although it got moved around multiple times since then.

Author: Christoph Berg <myon@debian.org>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/aFVR7R7VDX7y2ruc@msg.df7cb.de

src/makefiles/pgxs.mk

@@ -376,10 +376,7 @@ endif
 ifdef REGRESS
 # things created by various check targets
 	rm -rf $(pg_regress_clean_files)
-ifeq ($(PORTNAME), win)
-	rm -f regress.def
 endif
-endif # REGRESS
 ifdef TAP_TESTS
 	rm -rf tmp_check/
 endif