There is no need to reconstruct it from within the scripts since the
main script already constructed it once.
Drop the previously useless setting of BUILD_TARGET from various
scripts. The value was always overwritten at a later point.
No functional change intended.
Signed-off-by: Wei Liu <liuwe@microsoft.com>
(cherry picked from commit c7e51e51e58a7547b813f0fd245b5e61058854d6)