net/dnsfallback: disable recursive resolver for now

It seems to be implicated in a CPU consumption bug that's not yet
understood. Disable it until we understand.

Updates tailscale/corp#15261

Change-Id: Ia6d0c310da6464dda79a70fc3c18be0782812d3f
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f