Remove defensive assertion in `relay_key_is_unavailable_()`.

Remove defensive assertion in `relay_key_is_unavailable_()`.

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 tpo/core/tor#40848.