plpython: Don't create pgxsdir subdirectory in installdir target

As of db23464, we don't install
anything there anymore from plpython, so we don't need to create the
installation directory anymore.

Author: petere

src/pl/plpython/Makefile

@@ -110,7 +110,7 @@ $(OBJS): | submake-generated-headers
 install: all install-lib install-data
 
 installdirs: installdirs-lib
-	$(MKDIR_P) '$(DESTDIR)$(datadir)/extension' '$(DESTDIR)$(includedir_server)' '$(DESTDIR)$(pgxsdir)/src/pl/plpython'
+	$(MKDIR_P) '$(DESTDIR)$(datadir)/extension' '$(DESTDIR)$(includedir_server)'
 
 uninstall: uninstall-lib uninstall-data