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.

test_runner/fixtures/neon_fixtures.py

@@ -3539,7 +3539,6 @@ class Endpoint(PgProtocol, LogUtils):
         # and make tests more stable.
         config_lines = ["max_replication_write_lag=15MB"] + config_lines
 
-        config_lines = ["neon.primary_is_running=on"] + config_lines
         self.config(config_lines)
 
         return self