Remove unused DocumentMarker comparison operators

It was pointed out in a code review that these are unused:
https://codereview.chromium.org/2780313002/diff/20001/third_party/WebKit/Source/core/editing/markers/DocumentMarker.h#newcode166

BUG=707867

Review-Url: https://codereview.chromium.org/2785023002
Cr-Commit-Position: refs/heads/master@{#464908}
1 file changed