Remove contents: write permission from generate-constraints (#45558)

The write permission cannot be set for PRs from forks in the
call workflow - so we have to come back to implicit permissions
and make explicit permissions passing a bit differently.

.github/workflows/generate-constraints.yml

@@ -44,12 +44,6 @@ on:  # yamllint disable-line rule:truthy
         description: "Whether to use uvloop (true/false)"
         required: true
         type: string
-permissions:
-  # This permission is only active for "canary" builds and PRs from the main repo
-  # All fork PRs are not allowed to have write permissions and this one is automatically downgraded to read
-  # Branch protection also prevents from pushing to the "code" branches so we can safely use this one to
-  # Push constraints to "constraints" branches which are non-code branches and are not protected
-  contents: write
 jobs:
   generate-constraints:
     permissions: