rtwn: remove the conditional compilation around the sc_ht40 option.

This option stems from a bunch of issues a long time ago where HT40
support on some NICs is unstable - likely because we're not setting
up the RF/baseband correctly.

In any case, it doesn't need to be conditionally compiled anymore.
Leave it in, leave it off by default, and various chipset initialisation
paths can decide whether to enable it themselves.

Reviewed by: emaste