CLEANUP: ssl: fix wrong #else commentary

The else is not for boringSSL but for the lack of Client Hello callback.
Should have been changed in 1fc44d4 ("BUILD: ssl: guard Client Hello
callbacks with HAVE_SSL_CLIENT_HELLO_CB macro instead of openssl
version").

Could be backported in 2.4.

src/ssl_sock.c

@@ -2700,7 +2700,7 @@ allow_early:
 #endif
 }
 
-#else /* OPENSSL_IS_BORINGSSL */
+#else /* ! HAVE_SSL_CLIENT_HELLO_CB  */
 
 /* Sets the SSL ctx of <ssl> to match the advertised server name. Returns a
  * warning when no match is found, which implies the default (first) cert