More idiomatic fix: clear unused stuff asap.

1 job for master in 14 minutes and 34 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
failed #56392
fp
build-iris.dev

00:14:34

 
Name Stage Failure
failed
build-iris.dev Build
Makefile.coq:326: recipe for target 'all' failed
make[1]: *** [all] Error 2
Makefile:6: recipe for target 'all' failed
make: *** [all] Error 2

real 0m33.103s
user 0m50.604s
sys 0m2.204s
ERROR: Job failed: exit code 1