Remove g5k-api's UI

Since the UI is not actively maintained and not used, we decided to remove it.
3 jobs for ui-deletion in 21 minutes and 58 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Lint
passed #751356
grid5000-docker
rubocop

00:01:31

 
  Test
passed #751357
grid5000-docker
test-for-buster

00:09:27

85.67%
 
  Deb
passed #751358
grid5000-docker
deb-for-buster

00:11:00