x86: have .insn correctly consider AVX10.2's 256-bit embedded rounding

Deriving operand size may no longer assume 512-bit vector size when
embedded rounding is in use. In fact it was apparently wrong to do so
in the first place, as that's not correct for scalar insns. Drop the
rounding type check altogether; we fall back to EVEX.LIG when no
suitable operand was specified anyway, later in the function (and, btw,
similarly for VEX encodings).