[cr-buildbucket.cfg] Remove defunct luci_migrator_host entry.

This field is deprecated and has no effect:
https://chromium.googlesource.com/infra/luci/luci-go/+/master/buildbucket/proto/project_config.proto#423

R=tandrii@google.com

Change-Id: I728426a83017b1c09c90a7d63f21e39266eb7e32
Reviewed-on: https://chromium-review.googlesource.com/c/custom-tabs-client/+/2324725
Reviewed-by: Andrii Shyshkalov <tandrii@google.com>