fix staging site crash because of tmp dir in mu-plugs

[backamblock]

damn you are too fast :D just made a screenshot but didn't get to upload it🥇

[jessuppi]

Btw, I think there is a bug in ss-install-wordpress-mu-plugins when using custom plugins in ss-config that is causing that tmp folder to appear sometimes in the /mu-plugins/ folder, I haven't got around to checking it.

[backamblock]

hm maybe because you use regular rm without -r which will not delete directories

i'll send a commit shortly

[jessuppi]

rm is actually defined in ss-functions to behave like rm -rf

I've been in the process of converting these all to ss_rm instead after a guy on StackOverflow told me it was better practice.

[backamblock]

Pull request updated to use ss_rm 👍🏻