x86: figure braces aren't really part of mnemonics

x86: figure braces aren't really part of mnemonics

Instead they're separators for pseudo-prefixes. Don't insert them in
mnemonic_chars[], handling them explicitly in parse_insn() instead. Note
that this eliminates the need for another separator after a pseudo-
prefix. While maybe not overly interesting for a following real
mnemonic, I view this as quite desirable between multiple successive
pseudo-prefixes (bringing things in line with the other use of figure
braces in AVX512's zeroing-masking).

Drop the unused is_mnemonic_char() at this occasion.