Remove unused variables
Remove unused variables

This removes two unused variables from `serpent.py`. In the first case,
we still need the call for `compile_expr(...)`, since the result value
is modified by that call. In the second case, the variable is not needed
at all.

Part of #59

ddworken/pyethereum,holiman/pyethereum,ethermarket/pyethereum,ebuchman/daoist_protocol,pipermerriam/pyethereum,inzem77/pyethereum,shahankhatch/pyethereum,vaporry/pyethereum,shahankhatch/pyethereum,ethereum/pyethereum,ebuchman/daoist_protocol,karlfloersch/pyethereum,jnnk/pyethereum,ckeenan/pyethereum,harlantwood/pyethereum,karlfloersch/pyethereum,ethereum/pyethereum,pipermerriam/pyethereum,ethers/pyethereum