cmd,log,net,runtime: simplify string prefix and suffix processing
Use the TrimPrefix, TrimSuffix and CutPrefix to simplify the code.
Change-Id: I3e2b271ec0d3f9ce664b830e2b0c21ab47337ed0
GitHub-Last-Rev:
4bd1577d2496173800d609da75b08f6de177aba9
GitHub-Pull-Request: golang/go#68629
Reviewed-on: https://go-review.googlesource.com/c/go/+/601675
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>