CLEANUP: quic: remove the unused tx part of struct quic_cstream

CLEANUP: quic: remove the unused tx part of struct quic_cstream

struct quic_cstream carries a tx substructure with offset, sent_offset
and buf members, inherited from the qcs object it was derived from to
handle CRYPTO streams. Nothing ever reads them: the TX side of the
CRYPTO stream is handled by the tx crypto buffers of its encryption
level for the data, and by the stream descriptor attached to the
cstream. Only quic_cstream_new() sets the three members and no reader
exists anywhere in src/ or include/.

They were already unread when they were introduced (commit 7e3f7c47e
"MINOR: quic: New quic_cstream object implementation"), so let's remove
them along with their initialization.