luci-base: drop unused functions in luci.sys

Drop a number of redundant functions from luci.sys to shrink the code a bit:

 * luci.sys.net.arptable()   - replaced by luci.ip.neighbors()
 * luci.sys.net.routes()     - replaced by luci.ip.routes()
 * luci.sys.net.routes6()    - replaced by luci.ip.routes6()
 * luci.sys.net.deviceinfo() - replaced by nixio.getaddrinfo()
 * luci.sys.net.pingtest()   - no known user

Signed-off-by: Jo-Philipp Wich <jo@mein.io>