igb(4): Remove disconnected SYSCTL

The global hw.igb.rx_process_limit knob never was adhered to by the
in-tree version of this driver but similar functionality is available
via the device-specific dev.igb.N.iflib.rx_budget.

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

(cherry picked from commit f221d35be7a5749e2a1246c81adc9c03c1dc545c)