Remove primary_is_running (#8162)

This was a half-finished mechanism to allow a replica to enter hot
standby mode sooner, without waiting for a running-xacts record. It had
issues, and we are working on a better mechanism to replace it.

The control plane might still set the flag in the spec file, but
compute_ctl will simply ignore it.

libs/compute_api/src/spec.rs

@@ -96,12 +96,6 @@ pub struct ComputeSpec {
     // Stripe size for pageserver sharding, in pages
     #[serde(default)]
     pub shard_stripe_size: Option<usize>,
-
-    // When we are starting a new replica in hot standby mode,
-    // we need to know if the primary is running.
-    // This is used to determine if replica should wait for
-    // RUNNING_XACTS from primary or not.
-    pub primary_is_running: Option<bool>,
 }
 
 /// Feature flag to signal `compute_ctl` to enable certain experimental functionality.