Deprecate FunctionOf

[kianenigma]

Sadly, yes. We recently learned that the whole thing was useless since the scope in which the right hand side of #[weight = { ... }]was being evaluated in already had access to all the parameters. So this is useless, adds only noise. It should be deprecated.

• Mark FunctionOf as #[deprecated]
• Remove all users in substrate and polkadot

A good candidate for something to be done #6040.

[athei]

Split off the Origin part to: paritytech/polkadot-sdk#355

[kianenigma]

@athei I think this by itself would be mostly code removal and pretty trivial for substrate/polkadot. But, I'd surely flag it as clientnoteworthy, and maybe even consider communicating it in public channels since it might break other projects.