Remove defensive assertion in `relay_key_is_unavailable_()`. (for 0.4.8)

This patch removes a call to tor_assert_nonfatal_unreached() in relay_key_is_unavailable_() that is only called when Tor is compiled without relay support.

Unfortunately, the non-fatal assertion causes a BUG log message to appear for clients when they start up without relay support for each CPU worker we spawn. This makes it spotting issues during bootstrap harder particularly for our iOS developers.

Since the call sites to get_master_identity_key() handles NULL values already, we do not think this will be an issue later on.

Reported by Benjamin Erhart (@tla) from Guardian Project.

Fixes #40848 (closed).