cmd/dist: delete unnecessary dirs from GOROOT on completion

On my machine, these directories add up to 276mb
and account for 40% of the size of the GOROOT directory.

Once bootstrapping is complete, they are never used again.

Fixes #31851

Change-Id: Idbf8f21bae3d64655aa43761cc778677add6234a
Reviewed-on: https://go-review.googlesource.com/c/go/+/175377


Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>