Remove complex-proxy once it's available somewhere common

[JHopeCollins]

The complex_proxy module from was used to refactor the REXI code in #520. To avoid Gusto depending on asQ the whole module and tests were copied over from asQ. This duplicates code and means that any updates to the module need to be manually added in both repositories.

We're going to look at making complex_proxy available standalone without needing all of asQ (firedrakeproject/asQ#199). Once this is done Gusto should use this version and remove the local copy.