ci(buildkite): fix GPU tests not being run on Buildkite

Remove the extra stray `steps` that sneaked in to run GPU tests on
buildkite CI.

Fixes: 12ab042 ("ci: run Documentation builds on buildkite instead of GHA")

.buildkite/pipeline.yml

@@ -41,7 +41,6 @@ steps:
     # Don't run Buildkite if the commit message includes the text [skip tests]
     if: build.message !~ /\[skip tests\]/
 
-steps:
   - label: "Documentation"
     plugins:
       - JuliaCI/julia#v1: