Bug 1670357 - Remove `make` targets for cleaning: `clean`, `realclean`, `clobber`, `distclean`, `clobber_all`, `everything` r=firefox-build-system-reviewers,mhentges

The `clobber` targets are superseded by `mach clobber`, so we don't need them for any reason. The `clean` target is meant to get you to a post-`configure` state, but it doesn't really work, and if it's necessary for you to be in that state for some reason you can just clobber and re-`configure`, so it doesn't seem worth it to get it working again. Instead, delete all of them. Also delete `everything` which is not useful when `clobber` doesn't exist.

Differential Revision: https://phabricator.services.mozilla.com/D93514