Canonical EC signature check against r value in the signature might be unnecessary.

https://github.com/EOSIO/fc/blob/ff56d25e587f8131cfde06f1eb61baa899bc8a89/src/crypto/elliptic_common.cpp#L161

For canonical signature checks most standard ec libraries only enforce a low_s however the code referenced in this issue is enforcing a low_r (r being less than half of the curve order) as well. I'm wondering why that check is needed? Checking low_s should be enough to protect against signature malleability.

A clarification on why a low_r is being checked would be greatly appreciated, and if there is no need for it can that check be removed?