scons: Don't check for Python 2

scons: Don't check for Python 2

The build system will now refuse to build gem5 if Python 2.x is
detected. Remove Python 2 specific python-config variants from the
list of candidates we try.

Change-Id: Id59be4a2969ce180848e5df02afdfb4a5b8125c1
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/36535
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>