Remove useless check on supply cap

[QGarchery]

This check is useless because:

• at this point in the code, we know that suppliedAssets > 0
• later we check that supplyAssets + suppliedAssets <= supplyCap
• so after this last check we know that supplyCap > 0

I would even argue that the error returned is clearer: the supply cap (of 0) has been exceeded. Note that supplying 0 would not have made it revert, so the error UnauthorizedMarket doesn't give the full picture (it reverts because the amount is greater than the cap)