Backed out unify_time_units call in concatenate

In #1003 an `iris.util` function, namely `unify_time_units` was added to Iris, and called within `iris._concatenate.concatenate` as a helper function for occasions when concatenation would fail due to differing epochs in the time coords of the cubes to be merged. However this (as discussed post-merge in the comments of #1003) has had the undesirable side-effects of:

 * enforcing unifying time coord epochs when concatenating CubeLists, which may not be the wish of the user, and
 * meaning that even if the concatenate operation still fails the CubeList is irreversibly and inescapably changed.

As such it is considered that it would be much better to remove the automatic call of the util function from concatenate and add a note in the documentation for concatenate (and merge, which benefits in the same way from the provision of this util function) pointing users toward the util function.

Removing the util function call from concatenate means the epoch tests for concatenate no longer pass. As such the failing test is moved to a new integration test that explicitly calls the util function.