Simplify "fail high upon reduction" in null search

Do not use threat move to detect the condition. This
let us to retire the big allows() function.

Test at short TC was within 50% score:
LLR: -2.95 (-2.94,2.94)
Total: 38272 W: 7941 L: 7940 D: 22391

To be verified with reverse long TC

bench: 4191565
Merge branch 'master' into search
Revert "Don't IID when in check also in PvNodes"

This reverts commit 11ad7e33c28c0fa49be362b5e0f2bd517d29e3d7.
Fix static null move eval condition

We want to have more fail high at shallow depths
that at high depths (as is natural)

bench: 3719614