[ci] Update fuzz merkledb workflow to target master branch

When the active branch changed from dev to master this job appears to
have been overlooked because nobody was seeing the notifications of
its failure.

Author: maru-ava

.github/workflows/fuzz_merkledb.yml

@@ -15,8 +15,6 @@ jobs:
     steps:
       - name: Git checkout
         uses: actions/checkout@v4
-        with:
-          ref: 'dev'
       - name: Set up Go
         uses: ./.github/actions/setup-go-for-project
       - name: Run merkledb fuzz tests