hikey: factory-images: Remove ANDROID_TOP_DIR assignment

The script here incorrectly sets the ANDROID_TOP_DIR assignment,
which breaks the use case documented in README.

For building with a pre-built image, one should have the ANDROID_TOP_DIR set
by build/setenv.sh

Change-Id: If1caa3edad3ab597534a379e8952c7c6621dd2d8
Signed-off-by: John Stultz <john.stultz@linaro.org>
diff --git a/factory-images/generate-factory-images-package.sh b/factory-images/generate-factory-images-package.sh
index 9ee2ae8..67715ee 100755
--- a/factory-images/generate-factory-images-package.sh
+++ b/factory-images/generate-factory-images-package.sh
@@ -14,8 +14,6 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-ANDROID_BUILD_TOP=.
-
 source $ANDROID_BUILD_TOP/device/common/clear-factory-images-variables.sh
 # HiKey unfortunately can't use the ./generate-factory-images-common.sh script
 source $ANDROID_BUILD_TOP/device/linaro/hikey/factory-images/generate-factory-images-hikey.sh