doc: remove costlysetup example

It is not included with docs, and
`example/costlysetup/sub_a/test_quick.py::test_quick` sleeps for 5s,
slowing down `doctesting` unnecessarily.

doc/en/example/costlysetup/conftest.py

@@ -1,20 +0,0 @@
-import pytest
-
-
-@pytest.fixture(scope="session")
-def setup(request):
-    setup = CostlySetup()
-    yield setup
-    setup.finalize()
-
-
-class CostlySetup:
-    def __init__(self):
-        import time
-
-        print("performing costly setup")
-        time.sleep(5)
-        self.timecostly = 1
-
-    def finalize(self):
-        del self.timecostly

doc/en/example/costlysetup/sub_a/__init__.py

@@ -1 +0,0 @@
-#

doc/en/example/costlysetup/sub_a/test_quick.py

@@ -1,2 +0,0 @@
-def test_quick(setup):
-    pass

doc/en/example/costlysetup/sub_b/__init__.py

@@ -1 +0,0 @@
-#

doc/en/example/costlysetup/sub_b/test_two.py

@@ -1,6 +0,0 @@
-def test_something(setup):
-    assert setup.timecostly == 1
-
-
-def test_something_more(setup):
-    assert setup.timecostly == 1