[ripple/rippled] Simplify strHex: (#2668)

mellery451 commented on this pull request.

Thanks for taking on this cleanup. For your consideration: I was thinking it would be nice to get rid of those Slice/Blob overloads and I came up with this: https://github.com/mellery451/rippled/commit/defc35f4d92fe6aa3f961c5e3e4b5b82727fee5f (There might be better ways to implement the concept check)