ICU-21520 Fixes typo in name of test data file; removes a regex that

ICU-21520 Fixes typo in name of test data file; removes a regex that
evaluates to the empty string and the result files are missing the
perf test name as a result. E.g. '_Mar_16_155820.html' instead of
'unisetperf_Mar_16_155820.html'.
Not obvious why the regex was even created since it doesn't seem
necessary because the name of the caller doesn't require additional
parsing. The rpaticular code is from 2002, maybe changes in Perl
account for the issue(?)