Avoid set skip of NUMERAL

This even skips matching numerical constants in rewrite rules,
e.g.,
```
let QLANE = define
 `QLANE reg 8 ix = QREG' reg :> LANE_B ix /\
  QLANE reg 16 ix = QREG' reg :> LANE_H ix /\
  QLANE reg 32 ix = QREG' reg :> LANE_S ix /\
  QLANE reg 64 ix = QREG' reg :> LANE_D ix`;;
```

arm/proofs/decode.ml

@@ -1014,7 +1014,6 @@ let PURE_DECODE_CONV =
     custom_word_compute_add_convs rw;
     int_compute_add_convs rw;
     num_compute_add_convs rw;
-    set_skip rw `NUMERAL` (Some 0); (* always skip visiting the subexpr of NUMERAL *)
     add_thms [obind; LET_END_DEF] rw;
     (* Do not add _BITMATCH. These will be covered by conceal_bitmatch. *)
     add_conv (`_MATCH:A->(A->B->bool)->B`, 2, MATCH_CONV) rw;