Remove `this.category` backup assignment in `expandSiteException`

Other usage of this function is to expand sites returned by
`getChooserExceptionList`. `getChooserExceptionList` does not have
access to the content settings type on the C++ side, and the FE
components do not use the `category` field, so just removing the backup
seems to be the cleanest solution.

Change-Id: Ie932a8ab4b6b7ab91783eb3579e075238840c961
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5526882
Reviewed-by: Theodore Olsauskas-Warren <sauski@google.com>
Commit-Queue: Fiona Macintosh <fmacintosh@google.com>
Cr-Commit-Position: refs/heads/main@{#1300609}