Revert "build: Add missing guix-gc.timer file to binary tarball."

This reverts commit 69f6edc1a8.

The intention is good, but nodist_systemdservice_DATA are meant to be
disposable artefacts generated from corresponding ‘.in’ files.

etc/guix-gc.timer doesn't fit that description, breaking builds:

  $ make clean && make
  …
  make[2]: *** No rule to make target 'etc/guix-gc.timer', needed by 'all-am'.  Stop.

nix/local.mk

@@ -153,9 +153,8 @@ systemdservicedir = $(libdir)/systemd/system
 nodist_systemdservice_DATA =			\
   etc/gnu-store.mount				\
   etc/guix-daemon.service			\
-  etc/guix-gc.service				\
-  etc/guix-gc.timer				\
-  etc/guix-publish.service
+  etc/guix-publish.service			\
+  etc/guix-gc.service
 
 etc/%.mount: etc/%.mount.in	\
 			 $(top_builddir)/config.status