docs: clarify google pay requirements

[java-james]

The Flutter Stripe plugin already appears to include the required stripe-android SDK.
In which case, developers do not need to manually add:

dependencies {
    implementation 'com.stripe:stripe-android:17.1.1'
}

Removing this line from the docs avoids confusion and prevents potential
version conflicts. Gradle dependencies are handled automatically by the plugin,
as confirmed by ./gradlew app:dependencies:

+--- project :stripe_android
|    +--- com.stripe:stripe-android:21.26.+ -> 21.26.1
|    ...

Would appreciate someone double checking my assumption here upon review. Thanks.