Cleanup of .gitignore.
Cleanup of .gitignore.

I removed things we obviously don't need (hopefully I wasn't
overzealous). There may be some additional things that can/should be
removed.

R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/685403003

collinjackson/mojo,afandria/mojo,afandria/mojo,collinjackson/mojo,chinmaygarde/mojo,afandria/mojo,chinmaygarde/mojo,afandria/mojo,chinmaygarde/mojo,collinjackson/mojo,jianglu/mojo,jianglu/mojo,afandria/mojo,jianglu/mojo,collinjackson/mojo,afandria/mojo,jianglu/mojo,collinjackson/mojo,collinjackson/mojo,chinmaygarde/mojo,afandria/mojo,collinjackson/mojo,jianglu/mojo,afandria/mojo,jianglu/mojo,chinmaygarde/mojo,chinmaygarde/mojo,collinjackson/mojo,jianglu/mojo,collinjackson/mojo,chinmaygarde/mojo,chinmaygarde/mojo,chinmaygarde/mojo,jianglu/mojo