Remove deadcode

[philoserf]

• The only place TrimGOPATH was used was its test
• I found one old closed issue reference at frame.Format: %+s prints more than "path of source file relative to the compile time GOPATH" #99, and
• the code was introduced in pull request Introduce Stacktrace and Frame #37

I found no reason to keep the otherwise dead code.

[davecheney]

SGTM. Thanks.