feat: disable admin server

[peterldowns]

Following up @hslatman's comment on #62, this PR completely disables the Caddy admin server. The previous PR stopped using it as a certificate issuer's ask endpoint, but this PR actually disables the server entirely.

Testing

• manually confirmed that localias run does NOT bind to port 2019

$ localias start
...
$ localias status
daemon running with pid 29450
$ lsof -Pn | grep -E ':2019'

• confirmed that proxying requests still seems to work correctly
• confirmed that cert issuance for new aliases works correctly

[hslatman]

Nice 😄