Remove leftover special piplinks code

This has all been replaced by mappings in data/mappings/businesslink.csv and
data/mappings/businesslink_ukwelcomes.csv. This was done in:
e88b588cc3383ca1356ff353f56d94af1b28ab02
and:
39299ffa921c18e14fe0fdd2c734053bf24c59b6
Changes to BIS legacy sites configuration
Merge pull request #851 from rjc123/newsites

Changes to BIS legacy sites configuration
Merge pull request #853 from rjc123/newmappings

Configure thepensionservice.gov.uk TLD rewrite
Merge pull request #854 from rjc123/newmappings

Site configuration and reconfiguration
Merge pull request #855 from rjc123/newmappings

Change SACN Title
Change naturalengland TNA timestamp
Merge pull request #857 from alphagov/natural-england-timestamp

Change naturalengland TNA timestamp
Merge pull request #845 from alphagov/remove_leftover_special_piplinks_code

Remove leftover special piplinks code
Merge pull request #856 from rjc123/newmappings

Add mappings for missing authority IDs on worldpay callback
Document the optional `special_redirect_strategy` in the README
Reinstate old Worldpay callback proxy_pass

We didn't realise that this is a POST request which therefore can't be
redirected. Because they couldn't be redirected, the outcome of licencing
application payments via Worldpay isn't being recorded.

We are hoping to get Worldpay to POST to the new URL instead, but for now we
need to not break licence applications.
Merge pull request #860 from alphagov/reinstate_businesslink_ukwelcomes_worldpay_callback

Reinstate old Worldpay callback proxy_pass
Clarify the wording in the README about special_redirect_strategy

- Also use an em-dash to be consistent with the style of the other
  options.
Merge pull request #859 from alphagov/document_special_redirect_strategy

Document the optional `special_redirect_strategy` in the README
Add a special_redirect_strategy line to some of the site YAML files

- Only a few of them based on which sites have this field not set to nil
  in the transition database.
Update PHE_HPA's timestamp
Merge pull request #862 from alphagov/update_phe_hpa_timestamp

Update PHE_HPA's timestamp
Merge pull request #861 from alphagov/add_special_redirect_strategy_lines_to_sites

Add a special_redirect_strategy line to some of the site YAML files
Remove insignificant querystring params

Conflicts:
	data/transition-sites/hmrc_customs.yml