Remove custom concatenation of derived coords now that bug in iris is fixed

[schlunma]

For at least 6 years, iris dropped derived coordinates (i.e., aux factories) during concatenation (see SciTools/iris#2478). For this reason, we implemented our own custom code to solve this:

ESMValCore/esmvalcore/preprocessor/_io.py

Line 39 in 1f73cc0

def _fix_aux_factories(cube):

Since the issue is solved now since iris 3.5, we can remove our custom code.