Remove underline from AutoCompleteTextView via xml
This will also work properly
android:inputType="textNoSuggestions"
android:inputType="textPhonetic"
Just put:
android:background="@android:color/transparent"
inside your view.
EDIT: As @kiya said, nowadays is better to use
android:backgroundTint="@android:color/transparent"
As it maintains the view size.