Removal of legacy xtos code from main branch after v2.13 is forked.

[lgirdwood]

Now that everybody has either transitioned to Zephyr or is transitioning to Zephyr it makes good sense for future technical velocity to remove the legacy code, features and drivers associated with xtos.

Some of this work has already started with some vendors, but we should accelerate this after v2.13 to ensure we can execute new features with faster velocity (without having to consider impact/changes to legacy code).

I'd like to gather any feedback and opinions from others here before I create an issue where we can break down the work into smaller chunks and proceed with removals.

Removal of xtos from the main branch would not lead to a complete removal of xtos platforms from the codebase as they would still be available in the v2.13 stable branch which will soon fork for production release.

@thesofproject/amd @thesofproject/google @thesofproject/intel @thesofproject/mediatek @thesofproject/nxp