Regression since 78912688734589d67a9213201dbae45d231fad84,
where I removed the `matrix_hookshot_urlprefix` prefix group
`group_vars/matrix_servers`, thinking the value in `roles/custom/matrix-bridge-hookshot/defaults/main.yml`
was the same.

The value in `defaults/main.yml` incorrectly included `matrix_hookshot_public_endpoint`
in `matrix_hookshot_urlprefix`, which was leading to double-`/hookshot`-prefixing.

We were previously saved by the `matrix_hookshot_urlprefix` override in `group_vars/matrix_servers`.

This fix brings the correct URL prefix value (the one without `matrix_hookshot_public_endpoint`)
to `defaults/main.yml`.