Delete "SEBS" (aka: Clean up Kenton's old trash)

The file `SEBS` is a build file for a toy build system I was writing in my spare time six years ago, back when I maintained protobufs. Arguably it was never appropriate for me to have placed this file in the protobuf tree, but it was convenient at the time and didn't seem to hurt anything. It clearly doesn't belong here now, though.