Remove use MakeAssigneeList in webhooks to fix deadlock

[zeripath]

The webhooks currently use a call to MakeAssigneeList to get the assignees of an issue - however, this causes an SQL deadlock due to use of models.x whilst in a transaction. This call is unnecessary as the assignees are actually provided in the api.PullRequest.

Fix #6088

Signed-off-by: Andrew Thornton art27@cantab.net

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Ceterum autem censeo x exempla monstrabit esse delendam.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@8d3bb86). Click here to learn what that means.
The diff coverage is 0%.

@@            Coverage Diff            @@
##             master    #6102   +/-   ##
=========================================
  Coverage          ?   38.86%           
=========================================
  Files             ?      345           
  Lines             ?    49510           
  Branches          ?        0           
=========================================
  Hits              ?    19241           
  Misses            ?    27486           
  Partials          ?     2783

Impacted Files	Coverage Δ
models/webhook_dingtalk.go	0% <0%> (ø)
models/webhook_discord.go	1.39% <0%> (ø)
models/webhook_slack.go	0% <0%> (ø)

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