fix(pillarbox-monitoring): remove severity property from error event

Resolves #277 by removing the severity property following the update of the
monitoring specification.

Author: amtins

src/trackers/PillarboxMonitoring.js

@@ -248,7 +248,6 @@ class PillarboxMonitoring {
       message: error.message,
       name: error.code,
       ...playbackPosition,
-      severity: 'Fatal',
       url
     });