Remove session breadcrumbs

[mattjohnsonpint]

Session breadcrumbs were added in #1263 (released in 3.11.0).

Unfortunately, this creates some confusion such as seen in #2046.

• Ending a session typically occurs after an event would be captured, so an end-of-session breadcrumb is usually absent.
• Starting a session while one was already active would create an end-of-session breadcrumb followed by a new start-of-session breadcrumb, which often is confusing because they look out-of-sequence.
• AutoSessionTracking = true missed the start-of-session breadcrumb due to a bug (the hub it added the breadcrumb to is still disabled at that time).
• Pausing/Resuming Sentry sessions added breadcrumbs correctly, but they're still categorized as app.lifecycle - which is incorrect. The lifetime of the session does not necessarily correlate to the lifetime of the application. If app lifecycle breadcrumbs are desired, they should be added directly with the application events - not tied to Sentry sessions.
• We currently don't send an exited session update when AutoSessionTracking=true. The only benefit of adding that would be to enable session duration tracking, but that was removed from Sentry recently. See Release Health: Retiring Session Duration - Your Feedback Needed sentry#42716 (comment)

This PR removes session breadcrumbs. It does not change the behavior of sessions themselves.

It also adds some more diagnostic logging, and sets an appropriate level for existing logging messages.

(There's some minor code cleanup in here as well.)

Closes #2046