Remove default target configuration from integration test build script

Remove the handling of missing TARGET_PRODUCT, TARGET_BUILD_VARIANT, TARGET_RELEASE according to the build team's suggestion

Bug: 315143570
Test: TARGET_PRODUCT=aosp_x86_64 TARGET_RELEASE=trunk_staging TARGET_BUILD_VARIANT=userdebug tools/asuite/atest/integration_tests/atest_ci_tests.sh -b --artifacts_dir %dist_dir%
Change-Id: I8c16cf73e0e87414877441e22f0327adc4a1d5cd