chore(core): remove loop.chan

[M1nd3r]

All usage of loop.chan in the THP branch was replaced by the new loop.mailbox (introduced here: 9f01c9f). The loop.chan is no longer used in the main branch thanks to the RustLayout rework, Global layout object PR.

If there are no objections, the loop.chan can be removed from the codebase.

[github-actions]

legacy UI changes	device test(screens) main(screens)

[github-actions]

core UI changes	device test	click test	persistence test
T2T1 Model T	test(screens) main(screens)	test(screens) main(screens)	test(screens) main(screens)
T3B1 Safe 3	test(screens) main(screens)	test(screens) main(screens)	test(screens) main(screens)
T3T1 Safe 5	test(screens) main(screens)	test(screens) main(screens)	test(screens) main(screens)
All	main(screens)

[obrusvit]

The description of loop.chan from docs/core/src/event-loop.md could also be removed.

Ideally, we'd like a short documentation for loop.mailbox. Not necessarily in this PR.

[M1nd3r]

The description of loop.chan from docs/core/src/event-loop.md could also be removed.

Ideally, we'd like a short documentation for loop.mailbox. Not necessarily in this PR.

The description of loop.chan was removed and a (short) description of loop.mailbox added here 3ccc462