[config] remove url_backend_api

This can be derived from 'url' setting already.
4 jobs for 465-dev-share-url-fixes in 40 minutes and 15 seconds (queued for 6 seconds)
Status Job ID Name Coverage
  Cabal
passed #18559
cabal

00:18:57

 
  Stack
passed #18560
stack

00:02:31

 
  Bench
manual #18561
allowed to fail manual
bench
 
  Test
passed #18562
test

00:18:47