Remove rejected type ascription patterns from generated f*.

[maximebuyse]

Fixes #1170

This mostly ignores type ascription patterns in the F* backend (maybe we should completely ignore them, see #1170 (comment)). These patterns are translated as their subpattern. This fixes the issue from #1170

The main difference as a side effect is that for

let _: i8 = 1 + 1;

We will now generate

let _:i8 = 1y +! 1y in

instead of

let (_: i8):i8 = 1y +! 1y in

Both are valid F* and the new version is simpler, and clearer.

[W95Psp]

Thanks, that looks good to me!