gh-158816: Skip useless hash table probe when deleting from split dicts (#158819)

delitem_common() unconditionally called lookdict_index() to locate the index slot in dk_indices that points at the entry being removed.

That slot is only used by the combined branch, to overwrite it with DKIX_DUMMY.  In split tables the whole probe sequence was redundant work.

Move the call into the combined branch and drop the now-pointless assert() for split tables.

Microbenchmark (free-threaded build, -Og, no PGO; deleting keys from materialized instance dicts; medians over 12 alternating runs):

    del d[k]           71.2 -> 69.2 ns/op   (-2.8%)
    d.pop(k)           98.9 -> 96.1 ns/op   (-2.8%)
    colliding hashes   78.0 -> 74.4 ns/op   (-4.6%)