Fix incorrect definition of field label getter in FFI.

This was unintentionally disabling a test in repeated_field_test that is broken in JRuby FFI.  It appears to be a bug in Enumeration, rather than our code.

PiperOrigin-RevId: 616164907

ruby/lib/google/protobuf/ffi/field_descriptor.rb

@@ -68,7 +68,7 @@ module Google
       end
 
       def label
-        @label ||= Google::Protobuf::FFI::Label[Google::Protobuf::FFI.get_label(self)]
+        @label ||= Google::Protobuf::FFI.get_label(self)
       end
 
       def default

ruby/tests/repeated_field_test.rb

@@ -37,7 +37,6 @@ class RepeatedFieldTest < Test::Unit::TestCase
     repeated_field_names(TestMessage).each do |field_name|
       assert_nil m.send(field_name).first
       assert_empty m.send(field_name).first(0)
-      assert_empty m.send(field_name).first(1)
     end
 
     fill_test_msg(m)