Resolve a few Data warnings

[jmschonfeld]

Resolve a few warnings in Data code that snuck in recently.

1. The removed property is now unused and can be deleted
2. There are a few functions that I moved into the Data: MutableDataProtocol extension - these functions don't actually provide witnesses for MutableDataProtocol requirements but are close and thus produce compiler warnings about the slight mismatch. I split them out into a new extension to silence the warnings (but kept them in the same place organizationally)