Removal of curveFpDecodePointHex

The removal of this function is just a preliminary change to the total rework of the `ECKey` API.
It is currently not used by anywhere else but in `ec.js` as an almost negligible convenience function for initializing the curve parameters.

Other reasons for removing this function:
* It was not under test in any way
* It does not support EC point compression
* It has some weird return values