lte: Remove unnecessary fake header (commit a5fa8efd)

Commit a5fa8efd introduced a fake Rlc header only for the purpose of tagging it; however, the header is not necessary (the existing packet may be byte tagged). Tagging the first byte will result in the tag going out of scope when the existing header is removed (next layer up). If there is concern that there is no API to remove the byte tag without resorting to future header removal, I would instead suggest to work on this directly with new ns3::Packet API (by possibly invalidating ByteTagList::Item). But in this case, there does not seem to be any issue with just tagging the existing first byte.