scons: Eliminate CXX_V and main_dict_keys in SConstruct.

CXX_V isn't used by anything, and main_dict_keys is unnecessary because
using "in" with the whole main environment (which acts like a dict)
checks against the keys without needing a temporary variable.

Change-Id: Iab07246c00b1969858659043cead1dd657b1707b
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40963
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
1 file changed