chore(slack): remove handle dialog submission in webhook

[cathteng]

This is not being used.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 78.02%. Comparing base (7d1d769) to head (ac14116). Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #73519   +/-   ##
=======================================
  Coverage   78.01%   78.02%           
=======================================
  Files        6638     6638           
  Lines      296818   296803   -15     
  Branches    51115    51114    -1     
=======================================
+ Hits       231572   231577    +5     
+ Misses      58973    58955   -18     
+ Partials     6273     6271    -2     

Files	Coverage Δ
src/sentry/integrations/slack/webhooks/action.py	83.38% <ø> (+3.38%)	:arrow_up:

... and 4 files with indirect coverage changes