systemc: remove pipe through flag in TLM extension

systemc: remove pipe through flag in TLM extension

Pipe through flag should be equal to whether we have the extension
in TLM payload or not. However, in the current implementation the
two are different and cause issues when we have gem5 - SystemC
connection.

Change-Id: I2c318777d91dca446c1a700d9f7cff356d29ae6d
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/37375
Reviewed-by: Earl Ou <shunhsingou@google.com>
Maintainer: Earl Ou <shunhsingou@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>