rts: remove unused HAVE_C11_ATOMICS macro

This commit removes the unused HAVE_C11_ATOMICS macro. We used to have
a few places that have fallback paths when HAVE_C11_ATOMICS is not
defined, but that is completely redundant, since the
FP_CC_SUPPORTS__ATOMICS configure check will fail when the C compiler
doesn't support C11 style atomics. There are also many places (e.g. in
unreg backend, SMP.h, library cbits, etc) where we unconditionally use
C11 style atomics anyway which work in even CentOS 7 (gcc 4.8), the
oldest distro we test in our CI, so there's no value in keeping
HAVE_C11_ATOMICS.