Remove unneeded encode/decode methods.

In #110927 the encode/decode methods for `i8`, `char`, `bool`, and `str`
were made inherent. This commit removes some unnecessary implementations
of these methods that were missed in that PR.

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -108,11 +108,7 @@ impl<'a, 'tcx> Encoder for EncodeContext<'a, 'tcx> {
         emit_i64(i64);
         emit_i32(i32);
         emit_i16(i16);
-        emit_i8(i8);
 
-        emit_bool(bool);
-        emit_char(char);
-        emit_str(&str);
         emit_raw_bytes(&[u8]);
     }
 }