Remove redundant testutil files

Summary:
The only function in testutil.cpp, GetTempPath() simply called
fs::temp_directory_path() directly. This just tidies things up by
removing that redundant function and the file containing it.

Backport of PR11234

Test Plan:

make check
ninja check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2244