Copy the input slice in helper.SliceStringRemove()

[particleflux]

The previous version did, while returning a separate slice as output, also modify the original input slice. Deleting an element there then led to empty strings in the "unload --all" loop, when the loop slice args was implicitly modified.
This, in the end, lead to errors on unload --all when attempting to delete profiles named "".

e.g.:

$ env-manager unload --all
Unloading all profiles: awsMain,tfMysqlShared80Dev,allProxy,tfMysqlAuthDev

args: []string{"awsMain", "tfMysqlShared80Dev", "allProxy", "tfMysqlAuthDev"}

Unloading profile: awsMain
args: []string{"tfMysqlShared80Dev", "allProxy", "tfMysqlAuthDev", ""}

Unloading profile: allProxy
args: []string{"tfMysqlShared80Dev", "tfMysqlAuthDev", "", ""}

Unloading profile: 
Error: profile name cannot be empty

Solved by copying the input slice.