Commit 888745be16 (dev_disconnect:
remove @ marker.) in v0.11 in April 2022) removed the '@' marker from
our dev_disconnect code, but one test still uses it.
Refactoring this code made it crash on invalid input. The test
triggered a db issue which has been long fixed, so I'm simply removing
it.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>