Remove and ignore build artifacts

(Hello! This is my first PR against one of your projects. I couldn't find contib guidelines so feedback is very welcome. Thanks)

Build artifacts [usually don't belong in the repository](https://robinwinslow.co.uk/2013/06/11/dont-ever-commit-binary-files-to-git/).

This PR removes them and adds a [.gitignore](http://git-scm.com/docs/gitignore) file (taken from [github/gitignore](https://github.com/github/gitignore)) so they won't turn up again.

A similar change appears in many of this repo's forks curit/le_dotnet@e0c8c1b5fa54f33a96b5aa77844f80f545d3b0ee alxbog/le_dotnet@756dccc81ffedbde17f425c2cfa2108dc7f7a068 vlardn/le_dotnet@53844c8cea6fcdd714b84486268185ab5f488a8b nfleet/le_dotnet@9f99076e875c8f3ff7a5bf24b49019e5c5cacca8 mleyb/le_dotnet@a24f3485d7638777233003c9b907e6a6485674bd