[x86 testing] NFC: remove a few needless vector popcnt tests

The removed tests are handled by optimization passes before code gen and
therefore are just a distraction when making code gen changes that may
(as a side effect) reimplement earlier optimization work as a side effect.

Specifically, the following tests where removed:

ult_0_v* -> false
ult_1_v* -> x == 0
ugt_0_v* -> x != 0

ult_{size-of-element-plus-one}_v* -> true
ugt_{size-of-element}_v*          -> false

ult_{size-of-element}_v*           -> x != mask
ugt_{size-of-element-minus-one}_v* -> x == mask