stdlib, configs: Fix riscv-ubuntu-run error

This change,
https://gem5-review.googlesource.com/c/public/gem5/+/58790,
mistakenly constructs the root object outside of the
Simulator object, which would create another Root object.

The correct usage of a Simulator object is to pass the board to
the object without defining the root.

Change-Id: Iaa9aabe058a00abd964bf935b1b3f05456713275
Signed-off-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/58889
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
diff --git a/configs/example/gem5_library/riscv-ubuntu-run.py b/configs/example/gem5_library/riscv-ubuntu-run.py
index 1c95d3f..f3e6d13 100644
--- a/configs/example/gem5_library/riscv-ubuntu-run.py
+++ b/configs/example/gem5_library/riscv-ubuntu-run.py
@@ -115,7 +115,5 @@
     ),
 )
 
-root = Root(full_system=True, system=board)
-
 simulator = Simulator(board=board)
 simulator.run()