systemc: remove boost header dependency

systemc: remove boost header dependency

The current tests included don't require boost header to work. Remove
the dependency. This also gets rid of the warning message generated by
the latest boost headers.

Tested by running systemC tests:

src/systemc/tests/verify.py --update-json \
  --filter-file=src/systemc/tests/working.filt -j 56 build/ARM/

Change-Id: I9d3bfe145597335abdf24f2de85ed3c0708aea27
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40315
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>