Remove deprecated `kotlin.Experimental` opt-in

This would produce the following warning messages:

```
w: Opt-in requirement marker kotlin.Experimental is unresolved. Please make sure it's present in the module dependencies
```

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

@@ -62,7 +62,6 @@ internal fun Project.configureKotlinAndroid(
                 // Enable experimental coroutines APIs, including Flow
                 "-opt-in=kotlinx.coroutines.ExperimentalCoroutinesApi",
                 "-opt-in=kotlinx.coroutines.FlowPreview",
-                "-opt-in=kotlin.Experimental",
             )
         }
     }