Remove fallback to jwks when fetching jwk

[j3parker]

• Everyone supports JWK
• The fallback can cause perf issues due to the response size of jwks, its uncachability and a bug with one of the public key hosts in the system
• Let's keep everyone supporting jwk

[j3parker]

Verified okay for existed auth service clients?

Yep, a bit ago, but I'll revalidate today.

Also, we'll need to make a branch and cherry-pick this over, master isn't shippable (it is theoretically, but not practically)

D'oh, ok. Do you know what commit we should branch at?

[omsmith]

Likely candidates:

• 5f7e2b410bcb5794e3a5942b5edac78e534c6ea8 (7.2.2)
• a85271cffd78d4cb4036fb34949c587e430947fd (7.2.2 + formatting / code cleanups)

[j3parker]

I'll go with a85271c to avoid possible merge conflicts

[omsmith]

Made a branch 7.x with https://github.com/Brightspace/D2L.Security.OAuth2/commit/58d78092d914ac5f76affde9b364825054f263b8 cherry-picked. These apply cleanly on top.

[j3parker]

I can cherry-pick this tomorrow and we'll figure out how exactly we want to do this.