Remove ?A style string literals from string tests; #4303

We have a plan to obsolete this style in the future.

test/t/string.rb

@@ -717,6 +717,5 @@ end
 assert('String literal concatenation') do
   assert_equal 2, ("A" "B").size
   assert_equal 3, ('A' "B" 'C').size
-  assert_equal 3, (?A "#{?B}C").size
   assert_equal 4, (%(A) "B#{?C}" "D").size
 end