e1000(4): Remove disconnected SYSCTL

The global hw.em.rx_process_limit knob has been replaced by the device-
specific dev.IF.N.iflib.rx_budget along with the conversion to iflib(4).

While at it, remove the - besides initialization of tx_process_limit -
unused {r,t}x_process_limit members.

(cherry picked from commit 0d6d28ce5650d1cd23dbe4bbac87fb103b3e2d3d)