scons: Get rid of a redundant "Warning:" in the SConstruct.
scons: Get rid of a redundant "Warning:" in the SConstruct.

The "warning()" method already prints "Warning:", so putting it in the
message itself means it gets printed twice.

Change-Id: Ic157355958fdf56739f865a926ecba071bb25c5e
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27127
Reviewed-by: Nikos Nikoleris <b6a1b3cd9d36c44e9eeb16bc870df3cb2a0cf469@arm.com>
Maintainer: Bobby R. Bruce <b9dba908ffb6c6d8ffe449cc88b3709c13c485c2@ucdavis.edu>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5