README: remove pipeline status badge

Now using the proper gitlab badges settings.
5 jobs for master in 65 minutes and 44 seconds (queued for 41 seconds)
Status Job ID Name Coverage
  Lint
passed #875601
rustfmt

00:00:19

 
  Test
passed #875603
dist

00:08:01

passed #875604
flatpak

00:47:39

passed #875602
test

00:08:06

 
  Extras
passed #875605
clippy

00:06:33