[X86] Remove some uncovered and possibly broken code from combineZext.

This code matches (zext (trunc (setcc_carry))) -> (and (setcc_carry), 1)
but the code never checks what type we're truncating too. An and
mask of 1 would only make sense if the trunc was to MVT::i1, but
we didn't check for that.

I believe this code is a leftover from when i1 was a legal type.