tox: Remove 'commands_pre'

These were added in I5ce1385245c76818777aa34230786a9dbaf723e5 but are
not necessary since I680777e5406368d35366602ac37e029db91c8a5c, the
latter reverting most of the former. Removed them.

Change-Id: Ic820ac288cc8b97b417ba9fa25328c30c99562a3
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>

tox.ini

@@ -52,7 +52,6 @@ commands = bindep test
 # --ignore-missing-imports tells mypy to not try to follow imported modules
 # out of the current tree. As you might expect, we don't want to run static
 # type checking on the world - just on ourselves.
-commands_pre =
 commands =
   flake8 {posargs}
   mypy --ignore-missing-imports zuul
@@ -71,7 +70,6 @@ commands =
 [testenv:docs]
 deps = -r{toxinidir}/doc/requirements.txt
        -r{toxinidir}/test-requirements.txt
-commands_pre =
 commands =
   sphinx-build -E -W -d doc/build/doctrees -b html doc/source/ doc/build/html
 
@@ -79,7 +77,6 @@ commands =
 commands = {posargs}
 
 [testenv:nodepool]
-commands_pre =
 install_command = {[nodeenv]install_command}
 commands = stestr run --concurrency=1 --test-path ./tests/nodepool {posargs}