Code simplification: remove the CENT rule

Originally reported on Google Code with ID 594

```
The cent rule was removed in Bitcoin 0.9 and is no longer relevant. It complicates fee
calculation dramatically and is pointless, now. We should (carefully!) delete the code
that implements it.
```

Reported by `mh.in.england` on 2014-11-26 15:51:23

No files, tests, or entry points are named. Start by locating the CENT rule and the fee-calculation code, then trace all callers and existing fee-related tests before removing it. Done means the obsolete rule is gone without changing the remaining fee calculation behavior, with the relevant tests passing.

Written by the indexing model from the issue text.