ci: re-enable CODSPEED_PERF_ENABLED in walltime benchmarks

Drop the explicit CODSPEED_PERF_ENABLED=false override so the CodSpeed
action runs with its default perf integration.

Author: not-matthias

.github/workflows/codspeed.yml

@@ -92,8 +92,6 @@ jobs:
       - name: Run the benchmarks
         uses: CodSpeedHQ/action@main
         continue-on-error: ${{ matrix.valgrind != 'local' }}
-        env:
-          CODSPEED_PERF_ENABLED: false
         with:
           working-directory: bench
           mode: walltime