proto: Remove UpdateUserPlan message (#36268)

This PR removes the `UpdateUserPlan` RPC message.

We're no longer using the message after
https://github.com/zed-industries/zed/pull/36255.

Release Notes:

- N/A

crates/collab/src/llm.rs

@@ -1,9 +1 @@
 pub mod db;
-
-pub const AGENT_EXTENDED_TRIAL_FEATURE_FLAG: &str = "agent-extended-trial";
-
-/// The name of the feature flag that bypasses the account age check.
-pub const BYPASS_ACCOUNT_AGE_CHECK_FEATURE_FLAG: &str = "bypass-account-age-check";
-
-/// The minimum account age an account must have in order to use the LLM service.
-pub const MIN_ACCOUNT_AGE_FOR_LLM_USE: chrono::Duration = chrono::Duration::days(30);