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
07fb2d5e9d