Remove io and profiling from default GAP_PKGS_TO_BUILD, instead let gap-actions/process-coverage take care of them

[fingolfin]

See gap-actions/process-coverage#4

We only need those for processing coverage, so this makes sense, and saves a bit of time for packages don't needing those. It also is conceptually simpler: packages don't need to "remember" to add those to GAP_PKGS_TO_BUILD. Of course packages which currently specify such a GAP_PKGS_TO_BUILD will keep working for now.

(Some care must be taken so that the io and profiling packages themselves keep working, but I think that's not difficult to do, as long as one is aware).

[fingolfin]

Cloning and building packages has been removed in v3, in favor of leaving this to separate build steps in your own workflows (an example has been added in the manual).

The next release of gap-actions/process-coverage also should not need this anymore, see gap-actions/process-coverage#22