Replace some !rw_lock_own() assertions with the stronger
!btr_search_own_any(). Remove some redundant btr_get_search_latch()
calls.

btr_search_update_hash_ref(): Remove a duplicated assertion.

btr_search_build_page_hash_index(): Remove a duplicated assertion.
rw_lock_s_lock() asserts that the latch is not being held.

btr_search_disable_ref_count(): Remove an assertion. The only caller
is acquiring all adaptive hash index latches.