Avoid recently obsolete seq-contains in css-mode

* lisp/textmodes/css-mode.el (css--join-nested-selectors): Replace
recently obsolete seq-contains with new predicate seq-contains-p.
1 job for master in 52 minutes and 38 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
failed #1146
test-all

00:52:38

 
Name Stage Failure
failed
test-all Test
make[2]: *** [check-doit] Error 1
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make[2]: Leaving directory '/builds/emacs/emacs/test'
make[1]: *** [check-expensive] Error 2
Makefile:288: recipe for target 'check-expensive' failed
make[1]: Leaving directory '/builds/emacs/emacs/test'
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make: *** [check-expensive] Error 2
ERROR: Job failed: exit code 1