pkt-line.[ch]: remove unused packet_read_line_buf()

pkt-line.[ch]: remove unused packet_read_line_buf()

This function was added in 4981fe750b1 (pkt-line: share
buffer/descriptor reading implementation, 2013-02-23), but in
01f9ec64c8a (Use packet_reader instead of packet_read_line,
2018-12-29) the code that was using it was removed.

Since it's being removed we can in turn remove the "src" and "src_len"
arguments to packet_read(), all the remaining users just passed a
NULL/NULL pair to it.

That function is only a thin wrapper for packet_read_with_status()
which still needs those arguments, but for the thin packet_read()
convenience wrapper we can do away with it for now.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>