fix(HandleIDPInitiated): Remove recursive rlock

[dmoerner]

There is a recursive RLock in the path HandleIDPInitiated -> ServeIDPInitiated -> GetServiceProvider. This can lead to deadlocks under concurrent requests. The outer RLock is unnecessary, because GetServiceProvider is the only call to access the protected serviceProviders map, and it already guards access with RLock.

For further context from a similar issue, see: https://github.com/clerk/saml/commit/91716e8856a3a1723129577d6dfb5352d06fc837

A similar fix for a different recursive rlock was already merged into upstream here: https://github.com/crewjam/saml/pull/553