Don't dist SYMBOLS_ZIP in make on unbundled builds

Soong will now handle this. This requires that all unbundled builds
only build unbundled soong-built modules.

Also remove references to $(COVERAGE_ZIP) that I missed when I removed
its main build rule in ag/33260010.

Bug: 413145177
Test: COVERAGE_MODULES="ADBD" TARGET_RELEASE=mainline_2025_06 TARGET_BUILD_VARIANT=userdebug PRODUCT=mainline_modules_x86_64 MODULE_RELEASED_PLATFORM=R ./vendor/google/build/build_unbundled_coverage_mainline_module.sh, and then compared the disted ADBD-symbols.zip.
Change-Id: I92d8a687abb4aa7ce1fc6cb8979c75036eb3ab0a