Sets DOT asset rates for USDT and USDC. The asset rates were not mapped during the migration and therefore need to be redefined. Since the asset IDs (XCM locations) were not re-anchored, they cannot be used correctly in their current form. Whitelisted by [432](https://collectives.subsquare.io/fellowship/referenda/432).

## Context

Asset rates are used to verify the maximum spend amount allowed for each Governance track.
Every track has a maximum permitted spend defined in DOT terms, and the asset rates define the conversion of a given asset relative to them.

This batch includes calls to remove the unmapped asset rates and re-add them using the corrected asset IDs while preserving the same rate values.

Each rate is represented as a Fixed-Point 128-bit unsigned integer, using a divisor of 10¹⁸.
Internally, the type is a wrapper around u128 and decodes accordingly.

## Concrete Value

The constant value of `1,000,000,000,000,000,000,000` in the call is the same as before and calculated as:  
- DOT denomination: 1e10
- USDC (asset 1337) and USDT (asset 1984) denomination: 1e6
- Precision of the FixedU128 type that stores the value: 1e18

To calculate the ration for 10$ per DOT:
USDC per DOT = 1e10/((1e6*1,000,000,000,000,000,000,000)/1e18) = 10

#1787 Set DOT Asset Rates for USDT and USDC

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