glamor: Remove unused format_for_pixmap helper

Reviewed-by: Eric Anholt <eric@anholt.net>
2 jobs for glamor-render-formats in 1 minute and 32 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Docker Image
passed #68522
debian-testing

00:00:26

 
  Build And Test
passed #68523
build-and-test

00:01:06