Let amoc user processes exit normally

[NelsonVides]

As discussed in the internal communication channel, trying to report normal_user_stop still results in an abnormal exit, so this is buggy code, and altogether makes processes promoted to OTP gen processes to die with terrible crash reports. Logic is “call start, if it crashes, crash too”, so it makes sense to replace the throw in that amoc_user:stop/1 function with exit(shutdown) and not have any try ... catch.

[chrzaszcz]

Looks good 👍