Remove target FX from FSharp.CompilerBinding, allow compile on MD 5.5/5.6 and unused variable warnings

I was looking through the issues and saw #772 and thought I'd clean it up for you all.

This removes the "targetFramework" argument which was only used in one corner case to determine an FSharp.Core parameter for scripts where getting the project options for the script didn't give us an FSharp.Core.  Just setting it to .NET 4.5 in that case seems much better than plumbing an ignored parameter around

It also enable --warnon:1182 to give unused variable warnings and does some cleanup for that.