Remove second mypy run from pre-commit

[fjetter]

We introduced a second mypy check that runs in a platform compat mode for windows in #7094

Executing a single mypy check can take a significant amount of time. Below you can see the timings for me when running all hooks. As you can see, mypy takes about 15s on my machine. Adding an additional, mostly redundant 15s to every commit is not justified.
It's also worth pointing out that we're not running this for every python version even though mypy potentially is having the same problem there, see also python/mypy#10747

main ✔                                                                                                                                                                                        1d1h
▶ pre-commit run -av
absolufy-imports.........................................................Passed
- hook id: absolufy-imports
- duration: 0.3s
isort....................................................................Passed
- hook id: isort
- duration: 0.92s
pyupgrade................................................................Passed
- hook id: pyupgrade
- duration: 1.18s
black....................................................................Passed
- hook id: black
- duration: 0.17s

All done! ✨ 🍰 ✨
286 files left unchanged.

flake8...................................................................Passed
- hook id: flake8
- duration: 11.41s
codespell................................................................Passed
- hook id: codespell
- duration: 0.16s
mypy.....................................................................Passed
- hook id: mypy
- duration: 14.88s

Success: no issues found in 287 source files

mypy.....................................................................Passed
- hook id: mypy
- duration: 14.41s

Success: no issues found in 287 source files

cc @crusaderky you added this. Any strong objections?

[github-actions]

Unit Test Results

See test report for an extended history of previous test failures. This is useful for diagnosing flaky tests.

       15 files  ±0         15 suites  ±0   6h 39m 4s ⏱️ - 2m 23s
  3 225 tests ±0    3 137 ✔️  - 1    83 💤  - 1  5 ❌ +2 
23 845 runs  ±0  22 931 ✔️  - 2  908 💤  - 1  6 ❌ +3 

For more details on these failures, see this check.

Results for commit 44e970e. ± Comparison against base commit 2963bf1.

[crusaderky]

Your flake8 runtime is abnormal.
My timings on linux:

flake8...................................................................Passed
- hook id: flake8
- duration: 1.97s
codespell................................................................Passed
- hook id: codespell
- duration: 0.11s
mypy.....................................................................Passed
- hook id: mypy
- duration: 21.14s

Not sure if it's because you're running on MacOSX but I'd be surprised if that was the case.