Remove benchmarks (hasura#887)

<!-- The PR description should answer 2 (maybe 3) important questions:
-->

### What

These take ages to run, are slowing development down and not offering
the value they should. We should be benchmarking engine, but not like
this.

<!-- What is this PR trying to accomplish (and why, if it's not
obvious)? -->

<!-- Consider: do we need to add a changelog entry? -->

### How

Remove benchmarks CI job from Buildkite.

<!-- How is it trying to accomplish it (what are the implementation
steps)? -->

V3_GIT_ORIGIN_REV_ID: 30a2c9d5f6ba09f5319a07fe394db8becaa16b8e

v3/justfile

@@ -86,9 +86,6 @@ drill:
 coverage *filterexp:
   just docker_with_source_only bash coverage.sh '{{filterexp}}'
 
-generate-benchmarks gh-issue gh-token:
-  just docker_with_test_env bash benchmark.sh {{gh-issue}} {{gh-token}}
-
 run-local-with-shell:
   #!/usr/bin/env bash
   cargo run --bin custom-connector | ts "custom-connector:" &