glib: Remove Cocoa event loop patch

Partially addresses #323 (closed), hopefully helping make progress in !864 (merged).

Currently this patch blocks a long overdue, much needed glib update. It's been added a while ago to make sure any UI-related (GL mostly) APIs work on macOS, where a Cocoa event loop is needed for that to happen. However, workarounds in GStreamer elements (e.g. glimagesink) are present which currently mostly solve this issue for us. There were some reports (long ago) about these workarounds not being enough in some cases - like some permission prompts not showing up or vtdec misbehaving - but quite a lot has changed in macOS since then and these seem to work fine now.

This MR mainly aims to unblock the glib update, hopefully to happen in 1.22. In case anything is noticed to be broken after this patch is removed, a better workaround can quickly be made (a simple wrapper API, similar to what the Rust examples use, for example) later.