cmd/go/internal/cache: remove log.txt from the cache directory

Also remove an existing log.txt in the cache directory during
'go clean -cache' if it exists.

Fixes #25323

Change-Id: I1c6e20554db0edc6d850a7b4379d71ef5844eaea
Reviewed-on: https://go-review.googlesource.com/c/go/+/167741


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>