Remove unused boolean attribute
Remove unused boolean attribute

Was originally removed in https://android.googlesource.com/platform/frameworks/support/+/2f2b4ac338abb059272e2b9ad8a59402f31e20c8
Then was added back in https://android.googlesource.com/platform/frameworks/support/+/19e78d4fcde0ec98661d1c278878357475b2816e
to support pre-v13 devices (back when we went down to v7).

Now this attribute is not used anymore after the matching
change in the framework -
https://android.googlesource.com/platform/frameworks/base/+/f51fa7deac6f48bdcecfae76192d370d9d143e6e

Test: ./gradlew appcompat:appcompat:connectedCheck --info --daemon -Pandroid.testInstrumentationRunnerArguments.class=androidx.appcompat.app.AlertDialogTest
Change-Id: I53843435bb1e69b83eb84a9f11da33acac1e2bf9

androidx/androidx,AndroidX/androidx,AndroidX/androidx,androidx/androidx,androidx/androidx,androidx/androidx,androidx/androidx,AndroidX/androidx,AndroidX/androidx,AndroidX/androidx,androidx/androidx,androidx/androidx,androidx/androidx,androidx/androidx,AndroidX/androidx,AndroidX/androidx,AndroidX/androidx,AndroidX/androidx,androidx/androidx,AndroidX/androidx