scons: Remove a blanket USE_KVM disable check from SConstruct.

Whether to honor the USE_KVM setting can be handled in a more targeted
way down at individual consumers. They will know what ISA the host needs
to be for their particular model to make sense, where the SConstruct has
to enable or disable it for the entire build of gem5.

Change-Id: I9e82e232203834f6fe56d0ce5cf696809a0970bc
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/56188
Maintainer: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>

SConstruct

@@ -733,9 +733,6 @@ Build variables for {dir}:
     if env['USE_EFENCE']:
         env.Append(LIBS=['efence'])
 
-    if env['KVM_ISA'] != env['TARGET_ISA']:
-        env['USE_KVM'] = False
-
     # Save sticky variable settings back to current variables file
     sticky_vars.Save(current_vars_file, env)