cmd/go/internal/str,cmd/go/internal/work: remove redundant Contains

cmd/go/internal/str,cmd/go/internal/work: remove redundant Contains

"cmd/go/internal/str.Contains" does the same thing as the "slices.Contains".

The name "str.Contains" is also easily confused with "strings.Contains".

Given that the slices package is already used in the package,
replace "str.Contains" with "slices.Contains".

"str.Contains" is no longer used so just remove it.

Change-Id: I80572464bd17d4a60e7ff41db3a77c4d0bd03fa3
GitHub-Last-Rev: e74d333f0fd4b9558382e9dba0f77717089f45a9
GitHub-Pull-Request: golang/go#64136
Reviewed-on: https://go-review.googlesource.com/c/go/+/542416
Auto-Submit: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Mauri de Souza Meneguzzo <mauri870@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>