Remove remaining uses of ${COMPILER_FEATURES:Mc++11}

All supported compilers have C++11 support so these checks can be replaced
with MK_CXX guards.
See also https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=252759

PR: 252759
Reviewed By: emaste
Differential Revision: https://reviews.freebsd.org/D28234