Fix: Ethereum extrinsic size is accounted twice for proof size

[pLabarta]

Ethereum extrinsic size is accounted twice for proof size, this PR substracts that cost once.

[conr2d]

IIUC, the Ethereum extrinsic size is accounted for twice:

• Additional Gas is applied based on the Ethereum transaction size, which is then converted through GasWeightMapping.
• The size is also accounted for in SelfContainedCall's CheckWeight.

Is my understanding correct? Since SelfContainedCall isn’t designed to be used exclusively with pallet-ethereum, unconditionally subtracting it from actual_weight could result in unexpected behavior. Should we consider adding conditions or refining the logic here?

[boundless-forest]

#1490 (comment) Check this for some deep thought behind this change. I need more time to figure out if this is the best solution.