Complete rollout of Rust crypto stack

[richvdh]

Since matrix-org/matrix-react-sdk#12203 (Element Web 1.11.58), all new logins on all deployments of Element Web/Desktop have been using the Rust crypto stack.

In addition:

• As of Roll out Rust crypto to 60% of existing users of stable Element Desktop element-desktop#1706 (due for release next week), 60% of existing logins on stable releases of Element Desktop will have been migrated to Rust crypto.
• As of Roll out Rust crypto to 60% of existing users on app.element.io #27527, 60% of existing logins on the https://app.element.io deployment of Element Web will have been migrated to Rust crypto.

We have not seen any particular causes for concern. I therefore think it is time to complete the rollout. In particular:

• All remaining existing logins in Element Desktop should be migrated to Rust crypto.
• All remaining existing logins in all deployments of Element Web should be migrated to Rust crypto.

Part of #27001

[richvdh]

I think the best way to do this is just to update MatrixClientPeg.initClientCrypto so that, effectively, useRustCrypto is always true, and remove the feature_rust_crypto and RustCrypto.staged_rollout_percent settings.