systemc: Remove _m5.systemc passthroughs from SystemC_Kernel.

These functions are now exposed through m5.systemc and m5.tlm.

Change-Id: I9f519debbe7380ee38594badcc1146e66e15f8a8
Reviewed-on: https://gem5-review.googlesource.com/c/16570
Maintainer: Gabe Black <gabeblack@google.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>