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>
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