We use the TS_USE_CACHED_NETMAP knob to condition loading a cached netmap, but
were hitherto writing the map out to disk even when it was disabled. Let's not
do that; the two should travel together.
Updates #12639
Change-Id: Iee5aa828e2c59937d5b95093ea1ac26c9536721e
Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>