dist clean has logic to delete command binaries from the cmd
directories in cleanlist. However, these days the only binary it could
possibly remove is "$GOROOT/src/cmd/cgo/cgo". This is clearly no
longer necessary, so remove this stale code.

When this logic was originally introduced in CL 5622058, it was driven
by cleantab (not cleanlist), which contained all of the cmd
directories, which were legion at the time because this was the era of
the [568][acgl] toolchain. CL 9154 deleted cleantab, and did the same
clean walk over the "cmd/" directories listed in buildorder. However,
buildorder was a list of packages necessary to build cmd/go, so the
only "cmd/" directory in buildorder at the time was "cmd/go". Hence,
at that CL, dist started deleting only a "$GOROOT/src/cmd/go/go"
binary. The modern cleanlist was introduced in CL 76021, as a list of
packages containing "generated files and commands". The only "cmd/"
directory in cleanlist the whole time has been "cmd/cgo" (and I'm
honestly not sure why cmd/cgo is in there), so since that CL dist has
only deleted "$GOROOT/src/cmd/cgo/cgo".

Change-Id: I1915eb938d1a0e22ae6a64e7648a21894d3e6502
Reviewed-on: https://go-review.googlesource.com/c/go/+/501136


Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>