scons: Delete unnecessary imports from SConstruct.
scons: Delete unnecessary imports from SConstruct.

Also get rid of the joinpath and splitpath aliases for os.path.join and
os.path.split. The aliases are only slightly shorter, and obscure these
extremely common python methods.

Change-Id: I0784e2d8dbaacb0690a59e9fa8c8258c1df0e6c3
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40874
Reviewed-by: Gabe Black <d3e8afea784ab4a50a2dbf9149d2eaa8478e1cbc@gmail.com>
Maintainer: Gabe Black <d3e8afea784ab4a50a2dbf9149d2eaa8478e1cbc@gmail.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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