cmd/builder: delete

We do not have any remaining old-style builders that use cmd/builder.
They've all either migrated to use the new cmd/buildlet and coordinator,
or have stopped running.

As a result, cmd/builder is no longer needed and can now be deleted.

Fixes golang/go#21191

Change-Id: Ie51f7fa3a43a7c4799b614d65a9409e970c201d9
Reviewed-on: https://go-review.googlesource.com/c/161118
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: dmitshur