[MM-59483] Remove legacy preload and custom login code

[devinbinnie]

Summary

This PR removes code in the Desktop App that was left to support older versions of the Mattermost Server/Web App:

• Removed the legacy preload listeners using window.postMessage, needed for server versions <v9.4
• Removed the custom login code that allows the app to go out to another site and login with the app, needed for versions <v9.2.

What this does mean is that as of this change, Mattermost servers running versions under v9.4 will now have a reduced experience, and may or may not function at all. We will be dropping support for those server versions in the Desktop App officially as of v5.11.

This is being done to both reduce complication, improve security (in the case of custom logins) and potentially improve performance (in the case of the preload script changes)

Ticket Link

https://mattermost.atlassian.net/browse/MM-59843

Remove legacy code for older unsupported Mattermost servers

[devinbinnie]

@yasserfaraazkhan We can test this whenever, but since we're removing support for older versions, we should double check that the versions we support are all working well :) I ran E2E tests on my local machine and everything went okay - but a bit more thorough testing seems applicable here.

[github-actions[bot]]

Here are the test results below:

Test Summary for Linux on commit 3b51570f4a63024eddb739386647daa894d58d4b

New failed tests found on Linux:

• menu/view MM-T816 Toggle Full Screen in the Menu Bar

Test Summary for macOS on commit 3b51570f4a63024eddb739386647daa894d58d4b

All stable tests passed on macOS.