cmd/vendor, cmd/pprof: use golang.org/x/term directly

The cmd/pprof package currently uses golang.org/x/crypto/ssh/terminal
which - as of CL 258003 - is merely a wrapper around golang.org/x/term.

Thus, drop the dependency on golang.org/x/crypto/ssh/terminal and use
golang.org/x/term directly.

Change-Id: Ib15f1f110c338b9dba4a91a873171948ae6298a4
Reviewed-on: https://go-review.googlesource.com/c/go/+/304691


Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>