CI: Remove the pytest-benchmark plugin from the benchmarks workflow

[seisman]

The workflow runs (https://github.com/GenericMappingTools/pygmt/actions/runs/7383547777/job/20084932197) says:

 ============================= test session starts ==============================
  platform linux -- Python 3.12.1, pytest-7.4.4, pluggy-1.3.0 -- /usr/share/miniconda/bin/python
  codspeed: 2.2.0 (callgraph: enabled)
  CodSpeed had to disable the following plugins: pytest-benchmark
  cachedir: .pytest_cache
  benchmark: 4.0.0 (defaults: timer=time.perf_counter disable_gc=False min_rounds=5 min_time=0.000005 max_time=1.0 calibration_precision=10 warmup=False warmup_iterations=100000)
  Matplotlib: 3.8.2
  Freetype: 2.12.1
  rootdir: /home/runner/work/pygmt/pygmt
  configfile: pyproject.toml
  plugins: benchmark-4.0.0, codspeed-2.2.0, mpl-0.16.1
  collecting ... collected 697 items / 631 deselected / 6 skipped / 66 selected

It means the pytest-benchmark plugin is not needed.

Patches #2908.