global: disable signals

[slint]

With the new search and percolator implementation, there is no need to keep any of the signals-based code in the repo. We should deprecate/remove the following:

• OAISERVER_REGISTER_RECORD_SIGNALS and OAISERVER_REGISTER_SET_SIGNALS config variables
  • Cannot yet be done, since we rely on them to index/delete the OAISet percolator documents
• Extension methods that register/unregister these signals
• Receivers
• Tasks
• get_affected_records
• Unused functions from percolators.py