Add missing Concurrent::Map#delete and #delete_pair signatures (#988)

Author: bolshakov

gems/concurrent-ruby/1.1/_test/map.rb

@@ -15,3 +15,6 @@
 
 map.compute_if_present(:maybe_present) { |old_value| old_value + 1 }
 map.compute_if_present(:a) { |_old_value| nil }
+
+map.delete(:a)
+map.delete_pair(:b, 2)

gems/concurrent-ruby/1.1/map.rbs

@@ -48,6 +48,9 @@ module Concurrent
     def marshal_dump: () -> Hash[K, V]
     def marshal_load: (Hash[K, V] hash) -> self
 
+    def delete: (K key) -> V?
+    def delete_pair: (K key, V value) -> bool
+
     # undef :freeze
 
     def inspect: () -> String