Remove empty rules for check_elf_files.timestamp

If we don't have any new dependencies, or we never set up the
check_elf_files.timestamp rule, don't create an empty one now.

There were >7000 of these unused and empty rules in my AOSP
aosp_cf_x86_phone build.

This was initially brought to my attention by the new checks in
https://github.com/google/kati/pull/189

Test: diff out/build-aosp_cf_x86_phone.ninja, only removed phony rules
Change-Id: Ia43cbbd04df4a01d2182b14e3ccbe5d5ecbcabad