Remove unused extern crates

Latest versions of nightly rust compiler can detect unused extern
crates, and this commit removes those marked as unused.
5 jobs for master in 16 minutes and 35 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Lint
passed #304708
rustfmt

00:00:20

 
  Test
passed #304709
test 1.31

00:06:16

failed #304711
allowed to fail
test nightly

00:07:51

passed #304710
test stable

00:06:37

 
  Extras
passed #304712
clippy

00:08:22

 
Name Stage Failure
failed
test nightly Test
          collect2: error: ld returned 1 exit status


error: aborting due to previous error

error: Could not compile `examples`.

To learn more, run the command again with --verbose.
ERROR: Job failed: exit code 1