Remove unused exception parameter from hbt/src/perf_event/PerPerfEventsGroupBase.h

[r-barnes]

Summary: -Wunused-exception-parameter has identified an unused exception parameter. This diff removes it.

This:

try {
    ...
} catch (exception& e) {
    // no use of e
}

should instead be written as

} catch (exception&) {

If the code compiles, this is safe to land.

Reviewed By: dmm-fb

Differential Revision: D58830840