.github: drop branches filter with single asterisk from workflows (#17682)

Drop usage of the branches filter with a single asterisk as this matches
against zero or more characters but not a forward slash, resulting in
PRs to branch names with forwards slashes in them not having these
workflow run against them as expected.

Updates https://github.com/tailscale/corp/issues/33523

Signed-off-by: Mario Minardi <mario@tailscale.com>

.github/workflows/webclient.yml

@@ -3,8 +3,6 @@ on:
   workflow_dispatch:
   # For now, only run on requests, not the main branches.
   pull_request:
-    branches:
-      - "*"
     paths:
       - "client/web/**"
       - ".github/workflows/webclient.yml"