scripts: remove the external test

external.test connected the client and SFTP client to a host named by
WOLFSSH_EXTERNAL_HOST. It only ran when WOLFSSH_EXTERNAL_TEST was set,
which nothing in the tree or in CI does, so it always exited 77.

- delete scripts/external.test
- drop it from scripts/include.am

Author: ejohnstown

scripts/external.test

@@ -15,5 +15,5 @@ endif
 # app wasn't built.
 dist_noinst_SCRIPTS+= scripts/sshclient.test
 
-dist_noinst_SCRIPTS+= scripts/external.test scripts/fwd.test
+dist_noinst_SCRIPTS+= scripts/fwd.test
 EXTRA_DIST += scripts/fwd.test.expect