DEV: Drop support for deprecated `in:private` search filter.

lib/search.rb

@@ -688,7 +688,7 @@ class Search
       elsif word == 'in:all'
         @search_all_topics = true
         nil
-      elsif %w{in:private in:personal}.include?(word) # remove private after 2.4 release
+      elsif word == 'in:personal'
         @search_pms = true
         nil
       elsif word == "in:personal-direct"

spec/components/search_spec.rb

@@ -259,7 +259,7 @@ describe Search do
       expect(results.posts).to contain_exactly(reply)
 
       results = Search.execute(
-        'mars in:private',
+        'mars in:personal',
         search_context: post.user,
         guardian: Guardian.new(post.user)
       )
@@ -276,7 +276,7 @@ describe Search do
       TopicAllowedGroup.create!(group_id: group.id, topic_id: topic.id)
 
       results = Search.execute(
-        'mars in:private',
+        'mars in:personal',
         guardian: Guardian.new(user)
       )