Remove unnecessary FlowPreview opt-in

Author: SimonMarquis

build-logic/convention/src/main/kotlin/com/google/samples/apps/nowinandroid/KotlinAndroid.kt

@@ -86,7 +86,6 @@ private fun Project.configureKotlin() {
                 "-opt-in=kotlin.RequiresOptIn",
                 // Enable experimental coroutines APIs, including Flow
                 "-opt-in=kotlinx.coroutines.ExperimentalCoroutinesApi",
-                "-opt-in=kotlinx.coroutines.FlowPreview",
             )
         }
     }