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.

changes/ticket40848

@@ -0,0 +1,4 @@
+  o Minor bugfix (defensive programming):
+    - Disable multiple BUG warnings of a missing relay identity key when
+      starting an instance of Tor compiled without relay support.
+      Fixes bug 40848; bugfix on 0.4.3.1-alpha.

src/feature/relay/routerkeys.h

@@ -53,7 +53,6 @@ void routerkeys_free_all(void);
 static inline void *
 relay_key_is_unavailable_(void)
 {
-  tor_assert_nonfatal_unreached();
   return NULL;
 }
 #define relay_key_is_unavailable(type) \