src.opts.mk: drop C++11 compiler conditional from OPENMP

We no longer support older C++ compilers, so do not need to explicitly
test for C++11 support.

After r339946 we stopped running cc --version during cleandir/obj
stages, so stopped setting COMPILER_FEATURES. This in turn meant
lib/libomp was excluded from the clean stage in a normal buildworld
(i.e., one without -DNO_CLEAN), and this is what caused recent build
failures with errors about missing ittnotify_static.c.

This commit should obviate the need for the workaround committed in
r359083. Thanks to bdrewery for the insight and for pushing for a
correct fix. There are more cleanups to be done, but this change is
a simplification and an improvement over r359083.

Sponsored by: The FreeBSD Foundation