aarch64: Remove incorrect disassembly constraint

A check in print_insn_aarch64_word asserted that part of the encoding
space couldn't contain any valid encodings, and then returned ERR_NYI
("Not Yet Implemented", perhaps?) for these values.  However, some of
the new FEAT_MOP4 instructions will trigger the assert.  The check seems
to be outdated, and is clearly no longer valid, so it can just be
deleted.

Additionally, there are no other assignments of ERR_NYI, so delete all
remaining references to this error type.

include/opcode/aarch64.h

@@ -1283,7 +1283,6 @@ enum err_type
   ERR_OK,
   ERR_UND,
   ERR_UNP,
-  ERR_NYI,
   ERR_VFI,
   ERR_NR_ENTRIES
 };

opcodes/aarch64-asm.c

@@ -2695,7 +2695,6 @@ aarch64_opcode_encode (const aarch64_opcode *opcode,
 	{
 	case ERR_UND:
 	case ERR_UNP:
-	case ERR_NYI:
 	  return false;
 	default:
 	  break;
@@ -2711,7 +2710,6 @@ aarch64_opcode_encode (const aarch64_opcode *opcode,
     {
     case ERR_UND:
     case ERR_UNP:
-    case ERR_NYI:
       return false;
     default:
       break;

opcodes/aarch64-dis.c

@@ -4188,7 +4188,6 @@ print_aarch64_insn (bfd_vma pc, const aarch64_inst *inst,
       break;
     case ERR_UND:
     case ERR_UNP:
-    case ERR_NYI:
     default:
       break;
     }
@@ -4207,7 +4206,6 @@ print_insn_aarch64_word (bfd_vma pc,
       [ERR_OK]  = "_",
       [ERR_UND] = "undefined",
       [ERR_UNP] = "unpredictable",
-      [ERR_NYI] = "NYI"
     };
 
   enum err_type ret;
@@ -4229,18 +4227,10 @@ print_insn_aarch64_word (bfd_vma pc,
 
   ret = aarch64_decode_insn (word, &inst, no_aliases, errors);
 
-  if (((word >> 21) & 0x3ff) == 1)
-    {
-      /* RESERVED for ALES.  */
-      assert (ret != ERR_OK);
-      ret = ERR_NYI;
-    }
-
   switch (ret)
     {
     case ERR_UND:
     case ERR_UNP:
-    case ERR_NYI:
       /* Handle undefined instructions.  */
       info->insn_type = dis_noninsn;
       (*info->fprintf_styled_func) (info->stream,