Changes

gsup_client: disable gen_server event tracing

Even with the logging level set to info, we're still seeing this:

  *DBG* gsup_client got call ...
  *DBG* gsup_client sent ...

Those are messages printed by the gen_server module itself, and
as it turns out we explicitly enable printing them by passing
{debug, [trace]} to gen_server:start_link/3.

This kind of logging is way too verbose and not useful for a
regular user, so let's disable event tracing.

Change-Id: I9d0ca595fd6aab2e0e16944e31dde02f5783c772
Related: SYS#7415