Remove trace_patterns at the end of the session

Trace patterns (see erlang:trace_pattern/3) was left active after the
mockgyver session was finished. This could cause unexpected trace
messages to be received by subsequent test cases with trace
activiated. Trace patterns activated by mockgyver are now removed at
the end of the session.

Backwards incompatiblity: Trace patterns are removed even if they
where activated when mockgyver was started.