*/*: Remove compiler:openmp from Mk/Uses/compiler.mk

The option implied a dependency on gcc but clang got openmp support long ago.

Remove compiler:openmp from Mk/Uses/compiler.mk

For ports using USE=compiler:openmp, just remove it and make them build with
clang.
Fix conditionals when necessary
Bump PORTREVISION where appropriate

If problem arises, they can be addressed by using USE_GCC=yes

An update to the Porter's Handbook will follow.

Approved by: portmgr (bapt)
Differential Revision: https://reviews.freebsd.org/D31971