Deprecate `decl_*` macros

[KiChjang]

We've now basically migrated from the previous function-like decl_* macros into the #[pallet] attribute macros, and it's time that we start deprecating/removing the old decl_ macros.

[yourarj]

@KiChjang I would like to take it up.

[ggwpez]

There are two merge requests leading up to this https://github.com/paritytech/substrate/pull/12401 and https://github.com/paritytech/substrate/pull/12445 which are both stale&closed.
Maybe someone can give this another nudge? The old macros still pollute the code base.

[juangirini]

The deprecated Module type alias for Pallet should also be removed

[ggwpez]

The deprecated Module type alias for Pallet should also be removed

I am not familiar with this, but looks like it already is deprecated?! Since two years actually…
https://github.com/paritytech/substrate/blob/9ffd3d70bfc44ab306323e33bf7d234743268b03/frame/support/procedural/src/pallet/expand/pallet_struct.rs#L197

[juangirini]

I am not familiar with this, but looks like it already is deprecated?! Since two years actually…

Yes, it's already time to remove it :)

[ggwpez]

Okay so closing his and creating another one for removal.