sgi/build-test-secureboot: remove copying the prebuilt nor flash

It is not required to copy prebuilt nor flash image into prebuilt
folder as this can be done during the execute phase.

Change-Id: I35085e346bdd6e90cd55d1469d1c6dbf1e64850e
Signed-off-by: Jagadeesh Ujja <jagadeesh.ujja@arm.com>