Fix `clear_cache_on_start` in pystencils.json

Due two weird recursive dependencies `clear_cache` will not be
available at this point. In-lining this function will make
the config key `cache.clear_cache_on_start: true` work again.
13 jobs for fix-clear-cache-on-startup in 20 minutes and 40 seconds (queued for 2 minutes and 36 seconds)
Status Job ID Name Coverage
  Test
passed #535718
docker
build-documentation

00:00:56

passed #535715
docker
flake8-lint

00:09:19

passed #535445
AVX docker
latest-python

00:03:24

passed #535716
docker cuda
minimal-conda

00:00:20

passed #535717
docker cuda
minimal-sympy-master

00:02:24

passed #535446
win
minimal-windows

00:07:02

manual #535450
AVX cuda11 docker allowed to fail manual
pycodegen-integration
passed #535444
AVX cuda11 docker
tests-and-coverage

00:03:35

passed #535447
AVX cuda11 docker
ubuntu

00:02:27

failed #535452
docker
build-documentation

00:01:16

failed #535451
docker
flake8-lint

00:01:09

failed #535448
cuda docker
minimal-conda

00:01:01

failed #535449
cuda docker allowed to fail
minimal-sympy-master

00:01:04