tests: remove gc unit test
tests: remove gc unit test

After moving to md5 for cache files, gc logic became very straightforward,
and this unit test became irrelevant. We still have functional test for gc
though, so it is not like we're leaving gc untested. Probably worth it to
write simple unit test for it later.

Signed-off-by: Ruslan Kuprieiev <f6538312ce47017d734b1357aabc970ebf896e32@gmail.com>

dataversioncontrol/dvc,dmpetrov/dataversioncontrol,dmpetrov/dataversioncontrol,efiop/dvc,dataversioncontrol/dvc,efiop/dvc