k256: enable endomorphism optimizations by default; remove non-endomorphism code

[tarcieri]

The last of the GLV patents in the US (US7110538B2) expired on September 25th, 2020 (and the European patents have been expired for awhile). IANAL, but from what I'm gathering endomorphisms are no longer patent-encumbered. Relevant story:

https://www.btctimes.com/news/hal-finneys-proposal-for-optimizing-bitcoin-to-be-enabled-in-bitcoin-core

bitcoin-core/secp256k1 is now switching to the endomorphism implementation by default:

bitcoin-core/secp256k1#826

I think it would make sense for us to do the same.

cc @fjarri @tuxxy

[tarcieri]

#213 enabled the endomorphism-mul feature by default. Now we just need to deprecate/remove the feature and the non-endomorphism code 🎉

[tarcieri]

The non-endomorphism code was removed in #217.