Revert "test: Install bids-validator from bids-standard/bids-validator#261"

This reverts commit b4af21a.

The relevant PR has been merged into `dev`.

Author: effigies

.github/workflows/validate_datasets.yml

@@ -76,8 +76,6 @@ jobs:
       run: |
         git clone -b dev https://github.com/bids-standard/bids-validator/ ../bids-validator
         cd ../bids-validator
-        git fetch origin refs/pull/261/head
-        git checkout FETCH_HEAD
         deno compile -A -o $HOME/.deno/bin/bids-validator src/bids-validator.ts
 
     - name: Install BIDS validator (legacy)