Remove seal_ prefixes from FFI

[agryaznov]

Follow-up to substrate#11444

[athei]

From the CI:

ERROR: An unexpected import function was found in the contract Wasm: get_storage

I think we need a fix for cargo contract which expects that imports start with seal_.

[ascjones]

From the CI:

ERROR: An unexpected import function was found in the contract Wasm: get_storage

I think we need a fix for cargo contract which expects that imports start with seal_.

use-ink/cargo-contract#816