Removal of the *charge* portion of the economic model

[ureeves]

Summary

In the streamlining of the economic protocol described in dusk-network/rusk#1644, we've decided to do away with the charging portion, and as such we need to remove such functionality here as well.

Detailed Description (optional)

By merging #363 we introduced a charging component, recognized by the ABI, and formalized into an enum:

piecrust/piecrust-uplink/src/types.rs

Lines 162 to 169 in 8502e9e

	#[derive(Debug, Clone, Archive, Deserialize, Serialize, Eq, PartialEq)]
	#[archive_attr(derive(CheckBytes))]
	pub enum EconomicMode {
	Allowance(u64),
	Charge(u64),
	None,
	Unknown,
	}

We would like to remove the Charge variant, and any code relevant to it. This will mean slight changes in the ABI, particularly in which modes are recognized. The set_charge function should also be removed.