luci-app-qos: remove l7 options

Cleanup luci-app-qos, as it still contains references to l7 functionality
that has been removed both from kernel and from qos-scripts.

Qos config page has a non-functional "Service" option and every visit to
qos config page generates currently an error as no l7 files can be found.
   daemon.err uhttpd[2285]: find: /etc/l7-protocols/: No such file or directory

Patch removes "Service" option and the l7 file search.

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>