closedcaption: remove unnecessary extern crate declarations

The ones for gstreamer and glib are left there to avoid importing
the macros from weird paths, since the bindings are not yet ready
for this modules system
3 jobs for mcc in 3 minutes and 21 seconds (queued for 1 second)
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Status Job ID Name Coverage
  Lint
passed #71203
rustfmt

00:00:14

 
  Test
passed #71205
test nightly

00:03:07

passed #71204
test stable

00:02:33