Update Ractor#[]

* Delete the test
* Add deprecated mark

Author: soutaro

core/ractor.rbs

@@ -411,6 +411,7 @@ class Ractor
   # get a value from ractor-local storage for current Ractor Obsolete and use
   # Ractor.[] instead.
   #
+  %a{deprecated: Use Ractor.[] instead}
   def []: (interned sym) -> untyped
 
   # <!--

test/stdlib/Ractor_test.rb

@@ -93,17 +93,6 @@ class RactorInstanceTest < Test::Unit::TestCase
 
   testing "::Ractor"
 
-  def test_aref
-    omit "Accessing ractor local storage" if RUBY_VERSION >= "3.5"
-
-    r = Ractor.new {}
-    r['foo'] = 'bar'
-    assert_send_type "(String) -> untyped",
-                     r, :[], 'foo'
-    assert_send_type "(Symbol) -> untyped",
-                     r, :[], :foo
-  end
-
   def test_aset
     omit "Accessing ractor local storage" if RUBY_VERSION >= "3.5"