[chore](client): drop the unused cleartext manifest placeholder

The manifest sets android:usesCleartextTraffic explicitly (a LAN instance may
be plain HTTP), so the build-type placeholder no longer feeds anything and its
comment claimed a mechanism that is not the one in use.

android/app/build.gradle

@@ -101,12 +101,6 @@ android {
             // Caution! In production, you need to generate your own keystore file.
             // see https://reactnative.dev/docs/signed-apk-android.
             signingConfig signingConfigs.debug
-            // A self-hosted instance may legitimately be plain HTTP on a LAN
-            // (`http://192.168.x.x:8080`), and the dev default is
-            // `http://127.0.0.1:8080` via `adb reverse`. The RN template leaves
-            // this false outside debug, which would make a release build unable
-            // to reach any non-TLS instance at all.
-            manifestPlaceholders = [usesCleartextTraffic: "true"]
             minifyEnabled enableProguardInReleaseBuilds
             proguardFiles getDefaultProguardFile("proguard-android.txt"), "proguard-rules.pro"
         }