mesa/driver/haiku: Drop Mesa swrast renderer

mesa/driver/haiku: Drop Mesa swrast renderer

This just created extra upkeep and the push to move extern
C's into mesa code would mean a large number of extern's
in core Mesa driver interfaces. The Haiku Gallium renderers
are mostly insulated via the C-based Haiku state tracker.

As any future hardware support in Haiku will be gallium
based, lets just drop swrast.

Haiku has a Mesa 7.12 fork for gcc2 that uses swrast.

This commit fixes the last of the Haiku build issues.

Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>