[FRAME] Remove deprecated calls from treasury pallet

[muharem]

As part of the deprecation process #138 the next call needs to be removed:

• pallet_treasury.propose_spend
• pallet_treasury.reject_proposal
• pallet_treasury.approve_proposal

Scope:

• remove deprecated calls;
• set explicit coded indexes for Error and Event enums, remove unused variants and keep the same indexes for the rest;
• remove unused Config's type parameters;
• remove irrelevant tests and adopt relevant using old api;
• remove banchmarks for removed calls;

mentor: @muharem

[chungquantin]

Hi, I am keen on working on this issue

[chungquantin]

@muharem I have created a PR for this issue. Could you please help me to review? There is only only task I am not sure.

set explicit coded indexes for Error and Event enums, remove unused variants and keep the same indexes for the rest;

I am trying to find an example of other people works for explicit coded indexes used in this codebase but could not find one. Could you suggest me an approach for this?

[muharem]

@chungquantin just commented, let me know if something not clear yet

[shawntabrizi]

I would like to also include these changes into any new release of the treasury pallet: https://github.com/paritytech/polkadot-sdk/pull/3970/files

So that we have only one set of breaking changes to the treasury pallet.