strings: remove "a copy of the string" from ToUpper/ToLower comments

When string letters are all in lower/upper cases, both functions respectively
return original string.

Fixes #30987

Change-Id: Ie8d664f7af5e087f82c1bc156933e9a995645bf4
Reviewed-on: https://go-review.googlesource.com/c/go/+/171735


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>