ci: remove benchmarks

Both github actions and my own VPS have too much noise to be useful.

.github/workflows/benchmark.yml

@@ -1,38 +0,0 @@
-name: benchmark
-on:
-  push:
-    branches:
-      - v2
-  pull_request:
-    branches:
-      - v2
-
-jobs:
-  base:
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@master
-        with:
-          fetch-depth: 0
-      - name: Setup go
-        uses: actions/setup-go@master
-        with:
-          go-version: "~1.16"
-      - uses: WillAbides/benchdiff-action@main
-        with:
-          benchdiff_version: 0.7.0
-          status_sha: ${{ github.sha }}
-          status_name: bench-result
-          status_on_degraded: neutral
-          benchdiff_args: |
-            --packages ./...
-            --cpu=1,2
-            --count=10
-            --warmup-count=1
-            --warmup-time=10ms
-            --tolerance=10
-            --base-ref origin/v2
-            --debug
-            --benchmem
-            --geomean
-            --sort=name