[GH Actions] Remove pull_request trigger for Android/iOS workflows

Summary:
Context: https://phab.comm.dev/D6146#184347

These workflows depend on GitHub Secrets in order to execute properly.
However, secrets aren't available to workflows kicked off from PRs by default for obvious security reasons.

These runs will never succeed anyways, so we should just remove them.

Test Plan: NA

Reviewers: michal, tomek, marcin, jon, O2 Blocking Reviewers, ashoat

Reviewed By: jon, O2 Blocking Reviewers, ashoat

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D6150