Use new excludedPackages option from SpecialSource

Rather than excluding the bouncycastle and argo packages by hand:

MinecraftPortCentral/MCPC-Plus@e15b08fd97e654842b83a9837842f18507cd7c16

instead use the newly-added SpecialSourceMP <excludedPackages> option.
Allows using the unedited mappings from https://raw.github.com/agaricusb/MinecraftRemapping/c71945e6f89bf00a1ce3e3929ce07c019b19fc0e/1.5.2/cb2pkgmcp.srg