Removed module 'ralph.util.pricing' and the code that relates to it.

`ralph.util.pricing` was a really old piece of code, causing some really weird problems recently, like changing some fields on devices (e.g. `rack`). This pull request removes this module and the code that relates to it (e.g. tests).

It should be considered as a part of larger operation (removal of some old views from Ralph, like 'Costs', 'Prices' and 'Reports') - therefore, there are some places marked as `a leftover from ralph.util.pricing`, which should be re-examined during that operation.