Remove unnecessary assembly

[DimaStebaev]

https://github.com/skalenetwork/IMA/blob/1.3.4-beta.1/proxy/contracts/schain/KeyStorage.sol#L112

Using of pure assembly is dangerous and in the case can be easily replaced nether by pure solidity syntax or more robust openzeppelin wrapper.

Another even more dangerous using of assembly is never used and can be removed.