Remove deprecation of heartbeat_in_pthread

In some circumstances services can be executed outside of mod_wsgi and
in a monkey patched environment. In this context we need to leave the
possibility to users to execute the heartbeat in a green thread.

The heartbeat_in_pthread was tagged as depreacted few months and planned
for a future removal. These changes drop this deprecation to allow to
enable green threads if needed.

Closes-Bug: #1934937
Change-Id: Iee2e5a6f7d71acba70bbc857f0bd7d83e32a7b8c