Remove Service Templates from codebase

Description

Remove Service Templates from the codebase, including the disable_service_templates feature flag (#327436 (closed)) created in #326962 (closed).

Code to remove (incomplete list):

• Integration.for_template
• Integration.where(template: ...)
• Service.where(template: ...)
• spec/features/callouts/service_templates_deprecation_spec.rb

Notes on files to look at and searches to do

• app/models/integration.rb
• a global search for service_template revealed some other interesting portions of service_template code