[JIT] Remove captureposarg_n

MVM_args_get_pos_num returns a MVMArgInfo struct as a value, and the ABI
of doing so is underdocumented and inconsistent between POSIX and Win64.
So what worked on POSIX didn't work on windows.
These functions need to be cleaned up for the benefit of the JIT.

src/jit/graph.c

@@ -1701,7 +1701,6 @@ static MVMint32 consume_ins(MVMThreadContext *tc, MVMJitGraph *jg,
     case MVM_OP_capturehasnameds:
     case MVM_OP_captureposarg:
     case MVM_OP_captureposarg_i:
-    case MVM_OP_captureposarg_n:
     case MVM_OP_captureposarg_s:
     case MVM_OP_captureexistsnamed:
     case MVM_OP_capturenamedshash:

src/jit/x64/emit.dasc

@@ -993,26 +993,6 @@ void MVM_jit_emit_primitive(MVMThreadContext *tc, MVMJitCompiler *compiler, MVMJ
         |2:
         break;
     }
-    case MVM_OP_captureposarg_n: {
-        MVMint16 dst = ins->operands[0].reg.orig;
-        MVMint16 src = ins->operands[1].reg.orig;
-        MVMint16 pos = ins->operands[2].reg.orig;
-        | mov TMP5, qword WORK[src];
-        | test_type_object TMP5;
-        | jnz >1;
-        | cmp_repr_id TMP5, TMP6, MVM_REPR_ID_MVMCallCapture;
-        | jne >1;
-        | mov ARG1, TC;
-        | mov ARG2, aword CAPTURE:TMP5->body.apc;
-        | mov ARG3, qword WORK[pos];
-        | callp &MVM_args_get_pos_num;
-        | mov WORK[dst], RV;
-        | jmp >2;
-        |1:
-        | throw_adhoc "captureposarg_n needs a MVMCallCapture";
-        |2:
-        break;
-    }
     case MVM_OP_captureposarg_s: {
         MVMint16 dst = ins->operands[0].reg.orig;
         MVMint16 src = ins->operands[1].reg.orig;