Remove "dynamic + :set" category from documentation,
because all dynamic flags support ":set"; this is a
leftover of "static + :set".

Test Plan: make html

Reviewers: bgamari, mpickering

Reviewed By: mpickering

Subscribers: mpickering, osa1, rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4942