Dockerfile build fix: oniguruma doesn't have a distclean target

[pjeby]

I was trying to build a docker image from master (because I needed @base64d), and the build failed saying there was no "distclean" target for oniguruma. Removing the corresponding command from the Dockerfile fixed this; presumably the need to delete oniguruma build files is sufficiently met later in the Dockerfile, by the already-existing rm -rf oniguruma-5.9.6 command.

[coveralls]

Coverage remained the same at 79.932% when pulling e06ca5c on pjeby:patch-1 into c538237 on stedolan:master.

[pjeby]

Closing in favor of #1631