Fix Issue Unsubscription

[6543]

since #9535 will take it's time ...

a quick fix

how to reproduce the bug:

1. watch a repo
2. unwatch a issue of this repo
3. reload (issue is shown as subscibed - its not) -> if you try again you get a 500 :/

and for the api (witch use this function GetIssueWatchers too) https://github.com/go-gitea/gitea/blob/master/models/issue_watch.go#L116 take kare of it ...

[zeripath]

I'd recommend popping a comment on explaining why this one differs from the other watcher functions and needs to get the negative watchers too.

[zeripath]

otherwise LGTM

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@6842d55). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #9634   +/-   ##
=========================================
  Coverage          ?   42.22%           
=========================================
  Files             ?      582           
  Lines             ?    77103           
  Branches          ?        0           
=========================================
  Hits              ?    32557           
  Misses            ?    40546           
  Partials          ?     4000

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[6543]

ready to merge 🚀