refactor: remove receive() stubs

[wjmelements]

Reviewer @ZenGround0 @rvagg
filecoin-services uses the fvm-solidity dep here.
I want to update fvm-solidity to remove the need for the receive fallback function, which isn't necessary when you use FVMPay.
Previous versions needed receive because the FVMPay mock previously used payable.transfer, but after this fix it now uses vm.deal to mock the SEND filecoin method.

Changes

• update fvm-solidity dep
• remove receive
• add additional balance testing

[rvagg]

lgtm, nice work