[dart2js] Remove obsolete unittests.

Bug: dart-lang/sdk#33072

Change-Id: I0f6d5d2595fdb5ceecdbec68acdab987829be770
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/147780
Commit-Queue: Joshua Litt <joshualitt@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>

Author: joshualitt

tests/compiler/dart2js/dart2js.status

@@ -39,7 +39,6 @@ deferred/load_mapping_test: Slow, Pass
 end_to_end/dart2js_batch_test: Slow, Pass
 end_to_end/exit_code_test: Slow, Pass
 end_to_end/in_user_code_test: Slow, Pass
-end_to_end/show_package_warnings_test: Slow, Pass
 
 [ $checked ]
 codegen/value_range_kernel_test: Slow, Pass