cmd/tailscaled,types/logger: remove TS_DEBUG_MEMORY and associated logger

Commit e5a8cf3b1 added feature/runtimemetrics, which emits heap bytes
and total process memory as clientmetrics when the
NodeAttrEmitRuntimeMetrics capability is set. That subsumes the job of
the TS_DEBUG_MEMORY envknob, whose only effect is to prefix every log
line with Go heap+stack and Maxrss via logger.RusagePrefixLog.

Updates tailscale/corp#39434

Signed-off-by: Jordan Whited <jordan@tailscale.com>

cmd/tailscaled/tailscaled.go

@@ -460,9 +460,6 @@ func run() (err error) {
 		return nil
 	}
 
-	if envknob.Bool("TS_DEBUG_MEMORY") {
-		logf = logger.RusagePrefixLog(logf)
-	}
 	logf = logger.RateLimitedFn(logf, 5*time.Second, 5, 100)
 
 	if envknob.Bool("TS_PLEASE_PANIC") {