Remove duplicate SECURITY.md file

[johnbillion]

Following on from the comments in #541, the .github/SECURITY.md file should be removed.

[jeffpaul]

@johnbillion doesn't seem clear in that issue that there's agreement to remove it, though perhaps moved out of the /.github directory?

[kasparsd]

@jeffpaul there are two duplicate files currently — one in the root of the repo and one under .github. We should keep just the root one.