refactor(mixer): improve delays

[raphaelcoeffic]

This PR removes the distinction between following with respect to how delays are handled:

• a conditional mixer line (using a set of flight modes and/or conditional switch),
• a non-conditional one.

Fixes #4254

[philmoz]

Looks ok to me. I was looking at this yesterday and your solution is almost exactly the same as what I came up with.

One thing, not directly related to this issue, is that the 'stickIndex' variable is redundant.
Change line 794 to:
mixsrc_t srcRaw = md->srcRaw;
and you can delete stickIndex.

[raphaelcoeffic]

@pfeerick we should probably mention this in the release notes. Just in case someone had set something in the delays that was ineffective then, but would be now.

[3djc]

Looks good to me, I think time to rework fully the mixer is closing, no matter how scary

[gagarinlg]

Only with unit tests with close to 100% code coverage

[pfeerick]

Can't see anything broken here... only fixed 😁 Servo auto test delay/slow mixer, and various switch-based slow/delay with FM conditionals working as expected.