Temporarily disable the upstream branch for benchmarks

The benchmark script uses git worktree.
The upstream branch contains a ghcide submodule, which is not well supported by
worktree.
Once this PR has been merged and the upstream branch no longer contains a git
submodule, we can reenable it in the bench config

Author: pepeiborra

ghcide/bench/config.yaml

@@ -55,5 +55,5 @@ versions:
 # - v0.4.0
 # - v0.5.0
 # - v0.6.0
-- upstream: origin/master
+# - upstream: origin/master
 - HEAD