Refactor specs prevent_cross_database_modification_spec

Some time ago we made all specs default to preventing cross database
modification. Thus the `prevent_cross_database_modification` tag was
unecessary. This meant slightly re-organising the tests and removing 1
redundant test and re-writing one test slightly since it wasn't covered
elsewhere.

spec/support_specs/database/prevent_cross_database_modification_spec.rb

@@ -57,31 +57,6 @@ RSpec.describe 'Database::PreventCrossDatabaseModification' do
     include_examples 'successful examples'
   end
 
-  describe 'with_cross_database_modification_prevented block' do
-    it 'raises error when CI and other data is modified' do
-      expect do
-        with_cross_database_modification_prevented do
-          Project.transaction do
-            project.touch
-            pipeline.touch
-          end
-        end
-      end.to raise_error /Cross-database data modification/
-    end
-
-    it 'raises an error when an undefined gitlab_schema table is modified with another table' do
-      expect do
-        with_cross_database_modification_prevented do
-          Project.transaction do
-            project.touch
-            project.connection.execute('UPDATE foo_bars_undefined_table SET a=1 WHERE id = -1')
-          end
-        end
-      end.to raise_error /Cross-database data modification.*The gitlab_schema was undefined/
-    end
-  end
-
-  context 'when running tests with prevent_cross_database_modification', :prevent_cross_database_modification do
   context 'when both CI and other data is modified' do
     def run_queries
       project.touch
@@ -137,7 +112,6 @@ RSpec.describe 'Database::PreventCrossDatabaseModification' do
         end
       end
     end
-    end
 
     context 'when CI association is modified through project' do
       def run_queries
@@ -171,4 +145,15 @@ RSpec.describe 'Database::PreventCrossDatabaseModification' do
       end
     end
   end
+
+  context 'when some table with a defined schema and another table with undefined gitlab_schema is modified' do
+    it 'raises an error including including message about undefined schema' do
+      expect do
+        Project.transaction do
+          project.touch
+          project.connection.execute('UPDATE foo_bars_undefined_table SET a=1 WHERE id = -1')
+        end
+      end.to raise_error /Cross-database data modification.*The gitlab_schema was undefined/
+    end
+  end
 end