Removed upb_Decoder's stateful, bespoke logic for field lookup.

[copybara-service]

Removed upb_Decoder's stateful, bespoke logic for field lookup.

upb_Decoder used to have its own logic for finding a MiniTable field by number. Instead of using the standard upb_MiniTable_FindFieldByNumber() function, it used a custom linear search that cached the most recently seen field index. The theory behind this design was that field numbers are usually serialized in increasing order, so by linear searching from the last seen index, we would have a small expected number of elements to inspect before finding the matching one.

In practice, it appears that we can use the generic searching logic with no loss in performance. Removing the stateful search has several benefits:

1. This simplifies the decoder since we no longer have to preserve this extra state (last_field_index). Note that it was not even possible to put this variable into upb_Decoder because it is preserved per parsed message, so we previously needed a stack of last_field_index values. This CL lets us get rid of that completely.
2. This makes parsing performance less sensitive to whether field numbers are serialized in order.
3. This simplification will help as we refine the dispatch logic that switches between the standard and fasttable decoders.