application/x-apple-systemprofiler+xml: remove plist magic and root-XML
plist XML files are in no way exclusive to system profiler XML files, they're used for a bunch of things.
Because we don't have them associated with anything else, this is a recipe for false positives, such as JPEG's that embed plist XML content. Remove related magic and root-XML to avoid that.
Closes #169 (closed)