Remove the FormatSecurity diagnostic group from the FormatNonLiteral warning group.

The FormatSecurity warning group does not need to be a sub-group
of the FormatNonLiteral group.

Patch by Zach Davis!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@202114 91177308-0d34-0410-b5e6-96231b3b80d8