Revert "e2e: releases nightly (#5906)" (#6031)

This reverts commit 64961e2267, to see if it will make the workflow dispatch trigger reappear and fix our Slack notification link.

.github/workflows/e2e-nightly.yml

@@ -13,8 +13,6 @@ jobs:
       fail-fast: false
       matrix:
         group: ['00', '01', '02', '03']
-          # todo: expand to multiple versions after 0.35 release
-        branch: ['master', 'v0.34.x']
     runs-on: ubuntu-latest
     timeout-minutes: 60
     steps:
@@ -23,8 +21,6 @@ jobs:
           go-version: '1.15'
 
       - uses: actions/checkout@v2
-        with:
-          ref: ${{ matrix.branch}}
 
       - name: Build
         working-directory: test/e2e