Remove HTTP -> HTTPS redirect hack

HTTPS redirect for readthedocs now works as expected.

The current live site is redirecting HTTP -> HTTPS correctly and so we can remove the horrible hackery in docs/js/httpredirect.js:

$ http http://guidebook.devops.uis.cam.ac.uk/
HTTP/1.1 302 Found
CF-Cache-Status: DYNAMIC
CF-RAY: 5890aba40934e5a3-MAN
Connection: keep-alive
Content-Language: en
Content-Type: text/html; charset=utf-8
Date: Fri, 24 Apr 2020 14:56:21 GMT
Location: https://guidebook.devops.uis.cam.ac.uk/en/latest/
Server: cloudflare
Set-Cookie: __cfduid=d4befc7998d3d1fa516e47437f24363591587740181; expires=Sun, 24-May-20 14:56:21 GMT; path=/; domain=.guidebook.devops.uis.cam.ac.uk; HttpOnly; SameSite=Lax
Strict-Transport-Security: max-age=0
Transfer-Encoding: chunked
Vary: Accept-Language, Cookie
X-Backend: web00000c
X-Content-Type-Options: nosniff
X-Frame-Options: DENY
X-RTD-System-Redirect: True
X-Served: Django-Proxito
X-XSS-Protection: 1; mode=block
cf-request-id: 024e499a880000e5a36f18d200000001