[FEATURE] Remove bespoke proposal types from the Treasury module

[dokwon]

Problem definition

Two governance proposals, TaxRateUpdateProposal and RewardWeightUpdateProposal seeks to contain governance influence on the tax-weight and reward-weight parameters. This is no longer necessary and is leading to code bloat.

Feature specification

Remove TaxRateUpdateProposal and RewardWeightUpdateProposal from the treasury module - namely from https://github.com/terra-project/core/blob/master/x/treasury/handler.go

Additional context
Docs / clients also need to be updated accordingly