strings: make variable/type association consistent in function signatures

The type annotation of some trim functions are inconsistent with all
other function signatures of the strings package.

Example:
func TrimRight(s string, cutset string) string

To be:
func TrimRight(s, cutset string) string

Change-Id: I456a33287bfb4ad6a7962e30a6424f209ac320c1
Reviewed-on: https://go-review.googlesource.com/c/go/+/226339


Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>