build: do not set TARGET_ARCH to default

build: do not set TARGET_ARCH to default

This check is useless, scons already checks this and returns such an
error message:
Invalid value for option TARGET_ARCH: mips.  Valid values are: ['x86',
'x86_64', 'arm', 'arm-v7a', 'armeabi-v7a', 'arm64']

Change-Id: I3f7dd3b9fcae875ab20349203fb77537f24be763
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Reviewed-on: https://gerrit.iotivity.org/gerrit/15431
Reviewed-by: Phil Coval <philippe.coval@osg.samsung.com>
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Dave Thaler <dthaler@microsoft.com>