Shenandoah: Remove superfluous assert is ShBS::load_reference_barrier()

Some of heap walk related tests in nsk/jdi suite failed after concurrent weak reference processing.

Can be easily reproduced:
TEST_VM_OPTS="-XX:+UseShenandoahGC" make CONF=linux-x86_64-server-fastdebug run-test TEST=vmTestbase/nsk/jdi/ObjectReference/referringObjects/referringObjects001/referringObjects001.java