Replace `dvc exp gc` command with `dvc gc`

[daavoo]

Background: https://iterativeai.slack.com/archives/C01APS0FHDM/p1665658700964949

Summary:

• Extension doesn't provide a way of cleaning up the DVC cache. Removing experiments (either manually or with exp gc) does not clear the cache. Users might not be aware of the implications of not cleaning the cache (disk space usage, etc) and be confused by thinking that removing an experiment also cleans up space.

The current UI for exp gc is good, IMO, and could be reused as-is for dvc gc. We should find other ways to hint about cache cleaning (button in Source Tree view?)

• dvc exp gc will most likely be removed from DVC (https://github.com/iterative/dvc/issues/7991)
• Users already have many different ways in the UI to remove multiple experiments, exp gc don't add much value.