bors: Remove ci/hydra-eval from required statuses

Recently we've seen several Bors merge failures from ci/hydra-eval
that terminate with

    error: unable to fork: Cannot allocate memory

This works around the issue (#1888) by removing ci/hydra-eval from
statuses required for Bors to merge.

This status will instead be added as a requirement to the GitHub
branch protection rules of this repo.

If the Hydra eval fails the first time around for the Bors merge
commit, Bors won't fail because eval isn't required for Bors
anymore. Hydra will automatically fix the eval, likely on the second
pass, and then build the job at which point Bors will get the build
notification and merge assuming the build is fine (which it should be
if the PR CI status is already passed).

bors.toml

@@ -1,6 +1,5 @@
 status = [
   "buildkite/cardano-wallet",
-  "ci/hydra-eval",
   "ci/hydra-build:required",
 ]
 timeout_sec = 7200