Remove extra "rejected" events

[awrichar]

Only "message_rejected" will be emitted for rejected definition messages -
no special per-type event will be emitted.

This requires one extra lookup in syncasync to find rejected token pools,
but seems better for overall consistency.

[codecov-commenter]

Codecov Report

Merging #482 (23fbf61) into main (83c265f) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #482   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          266       266           
  Lines        15154     15147    -7     
=========================================
- Hits         15154     15147    -7     

Impacted Files	Coverage Δ
pkg/fftypes/event.go	100.00% <ø> (ø)
...ternal/definitions/definition_handler_contracts.go	100.00% <100.00%> (ø)
...ternal/definitions/definition_handler_tokenpool.go	100.00% <100.00%> (ø)
internal/syncasync/sync_async_bridge.go	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 83c265f...23fbf61. Read the comment docs.

[awrichar]

Note that this change was prompted after drawing this picture of events: