Clean up dead code in hsh_deref_objhead_unlock
Clean up dead code in hsh_deref_objhead_unlock

req's on waiting list do indeed hold an objhead ref (req->hash_objhead),
so the condition for the while loop in hsh_deref_objhead_unlock will
always evaluate to false.

This gets rid of the needless rushing code and replaces it with an
assert.

gquintard/Varnish-Cache,gquintard/Varnish-Cache,gquintard/Varnish-Cache,gquintard/Varnish-Cache