mruby-compiler: remove unused node types from AST enum

Removes NODE_CDECL, NODE_CVASGN, NODE_CVDECL, NODE_ITER, and NODE_WHEN
from the node type enum as these node types are not used in the current
parser implementation.

Co-authored-by: Claude <noreply@anthropic.com>

mrbgems/mruby-compiler/core/node.h

@@ -13,12 +13,10 @@ enum node_type {
   NODE_BLOCK,
   NODE_IF,
   NODE_CASE,
-  NODE_WHEN,
   NODE_WHILE,
   NODE_UNTIL,
   NODE_WHILE_MOD,
   NODE_UNTIL_MOD,
-  NODE_ITER,
   NODE_FOR,
   NODE_BREAK,
   NODE_NEXT,
@@ -33,9 +31,6 @@ enum node_type {
   NODE_NOT,
   NODE_MASGN,
   NODE_ASGN,
-  NODE_CDECL,
-  NODE_CVASGN,
-  NODE_CVDECL,
   NODE_OP_ASGN,
   NODE_CALL,
   NODE_SCALL,