codegen: remove Bool special case in emit_unbox
Now that Bool is represented as i1 natively, the special case in
emit_unbox that loaded as i8 with !range metadata and then truncated
to i1 is no longer needed. The general load path handles Bool correctly:
`load i1, ptr` internally loads a byte and masks to 1 bit, which is
equivalent to the old manual approach.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
