Remove previously deprecated features (#1777)

- Treating multi-unit strings as numbers
- `rgbasm -Wnumeric-string`
- `ldio [c], a` and `ldio a, [c]` (use `ldh`)
- `ld [c], a` and `ld a, [c]` (use `ldh`)
- `ldh [$xx], a` and `ldh a, [$xx]` (use `$FFxx`)

test/asm/db-dw-dl-string.asm

@@ -11,7 +11,3 @@ SECTION "Test", ROM0
 	db "A" + 1
 	dw "A" + 1
 	dl "A" + 1
-
-	db 1, ("WXYZ") & $ff, -1
-	dw 1, ("WXYZ") & $ffff, -1
-	dl 1, ("WXYZ"), -1