Don't return NULL in createRemovalPolicy() if there is no match for

Don't return NULL in createRemovalPolicy() if there is no match for
the given policy type. None of the FS code handles this correctly,
and it was causing SIGSEGVs when a cache_replacement_policy was not
set in squid.conf, defaulting to lru, and lru was not compiled in.

We now warn very loudly and die with an error. cache.log will tell the
user to check cache_replacement_policy and memory_replacement_policy
in their squid.conf file.