cmd/builder: delete unused gccgo, old^2 perf dashboard, and mercurial support

Step 1 of migrating cmd/builder users to cmd/buildlet users: delete
unused code to make sense of what actually remains.

Updates golang/go#21191

Change-Id: I74d52d49ea0f3ff347ff188dcc0f2c491afca2ff
Reviewed-on: https://go-review.googlesource.com/51830
Reviewed-by: Andrew Bonventre <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Reviewed-by: Herbie Ong <[email protected]>

Author: bradfitz