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/invalid-ldh.asm

@@ -0,0 +1,3 @@
+SECTION "test", ROM0
+ldh [$11], a
+ldh a, [$22]