docs: remove bugsnag-em from the docs

[tnir]

Goal

bugsnag gem 5.x is no more used and so bugsnag-em should be unreferenced.

Design

As per https://github.com/bugsnag/bugsnag-em/pull/5, we should remove references for bugsnag-em gem.

Changeset

• Remove an entry about bugsnag-em from the docs.

Testing

n/a

[tnir]

@imjoehaines Status?