This reverts commit c8c6a83ecc.
Synapse v1.142.0 is broken for Matrix Authentication Service deployments
that use a path-prefix (e.g. `/auth`) such as ours.
For such deployments, Synapse fails to contact MAS at the correct
introspection endpoint (it keeps hitting `/oauth2/introspect`, instead of `/auth/oauth2/introspect`)
and is not usable.
Related to 3595ff921f