Revert "follow #16009 VM supports cast nil to ptr (#16012)"

This reverts commit fec19c980e.

compiler/vmgen.nim

@@ -901,9 +901,6 @@ proc genCastIntFloat(c: PCtx; n: PNode; dest: var TDest) =
     c.gABx(n, opcSetType, dest, c.genType(dst))
     c.gABC(n, opcCastIntToPtr, dest, tmp)
     c.freeTemp(tmp)
-  elif src.kind == tyNil and dst.kind in PtrLikeKinds:
-    if dest < 0: dest = c.getTemp(n[0].typ)
-    genLit(c, n[1], dest)
   else:
     # todo: support cast from tyInt to tyRef
     globalError(c.config, n.info, "VM does not support 'cast' from " & $src.kind & " to " & $dst.kind)