symbol.c: remove `Symbol#id2sym`.

In the ancient Ruby, symbols are represented by integers. In that era,
to get string representation from integers, we used `Integer#id2sym`
method. Later, `Symbol` was introduced, and `id2sym` was used for
compatibility. Today, no one uses `id2sym` any longer. It is described
in ISO 30170:2012 standard but I consider it as a mistake.

src/symbol.c

@@ -657,7 +657,6 @@ mrb_init_symbol(mrb_state *mrb)
   MRB_SET_INSTANCE_TT(sym, MRB_TT_SYMBOL);
   mrb_undef_class_method(mrb,  sym, "new");
 
-  mrb_define_method(mrb, sym, "id2name", sym_to_s,    MRB_ARGS_NONE());          /* 15.2.11.3.2 */
   mrb_define_method(mrb, sym, "to_s",    sym_to_s,    MRB_ARGS_NONE());          /* 15.2.11.3.3 */
   mrb_define_method(mrb, sym, "to_sym",  sym_to_sym,  MRB_ARGS_NONE());          /* 15.2.11.3.4 */
   mrb_define_method(mrb, sym, "inspect", sym_inspect, MRB_ARGS_NONE());          /* 15.2.11.3.5(x) */

test/t/symbol.rb

@@ -17,10 +17,6 @@ assert('Symbol#===', '15.2.11.3.1') do
   assert_false :abc === :cba
 end
 
-assert('Symbol#id2name', '15.2.11.3.2') do
-  assert_equal 'abc', :abc.id2name
-end
-
 assert('Symbol#to_s', '15.2.11.3.3') do
   assert_equal  'abc', :abc.to_s
 end