Remove unused FilterOperation::clone() functionality
Created attachment 143107 [details] Patch
This patch will only make sense after bug 87046 lands.
Custom Filters related code, which this patch was trying to modify was removed in below commit: https://github.com/WebKit/WebKit/commit/f163ea7cac88d117b8953ce2e3967a7e09bbf457 Nothing to do here.