misc: Remove raven-js package

We track raven-js (v3 series of JS SDKs) and release important bugfixes
only from the '3.x' branch, and never from master.

Over time, the code in raven-js in master and in the 3.x branch got out
of sync.

Removing raven-js from master eliminates confusion and cleans up the
workspace when developing the current 5.x series of packages.

Author: rhcarvalho