rts: Don't default to single capability when profiled

This was presumably a vestige of the days when the profiled RTS couldn't
run threaded. Fixes #14545.

Test Plan: simonmar

Reviewers: erikd, simonmar

Reviewed By: simonmar

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #14545

Differential Revision: https://phabricator.haskell.org/D4245

(cherry picked from commit 4bfff7a5)