[crypto] remove obsolete comment (enum code for multiple signing scheme variants is now macro-generated)
[crypto] remove obsolete comment
(enum code for multiple signing scheme variants is now macro-generated)

Closes: #1314
Approved by: metajack

aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,libra/libra,libra/libra,libra/libra,aptos-labs/aptos-core