* accounts/scwallet: use go-ethereum crypto instead of go-ecdh

github.com/wsddn/go-ecdh is a wrapper package for ECDH functionality
with any elliptic curve.

Since 'generic' ECDH is not required in accounts/scwallet (the curve is
always secp256k1), we can just use the standard library functionality
and our own crypto libraries to perform ECDH and save a dependency.

* Update accounts/scwallet/securechannel.go
Co-authored-by: Guillaume Ballet <gballet@gmail.com>

* Use the correct key
Co-authored-by: Guillaume Ballet <gballet@gmail.com>