Re-enable encrypt/decrypt on keypair

[davidrhodus]

• I'm submitting a request to revet the changes in https://github.com/polkadot-js/common/pull/1762 to Re-enable encrypt/decrypt on keypair

• [ ] Bug report
• [ ] Feature request
• [X] Support request
• [ ] Other

• What is the current behavior and expected behavior? Our application is expecting the encrypt/decrypt to be on the keypair object

• What is the motivation for changing the behavior?

We currently utilize encrypt/decrypt within out Substrate based project. We are requesting this feature be added back into the polkadot.js code.