cmake: partial revert of BOOST_USE_VALGRIND when ALLOCATOR=libc

cmake: partial revert of BOOST_USE_VALGRIND when ALLOCATOR=libc

the WITH_SYSTEM_BOOST binaries are not built with BOOST_USE_VALGRIND, so
it probably isn't safe to define for the headers only

this flag is needed for teuthology testing, and the shaman builds use
WITH_SYSTEM_BOOST=OFF. so the better fix is to enable WITH_BOOST_VALGRIND
so BuildBoost.cmake will build the libraries with valgrind support and add
-DBOOST_USE_VALGRIND to the necessary targets

this change was merged in https://github.com/ceph/ceph-build/pull/1736

Fixes: https://tracker.ceph.com/issues/48963
Signed-off-by: Casey Bodley <cbodley@redhat.com>