Cleanup proguard configuration

- Remove custom KotlinX Serialization config as it's already provided, see https://github.com/Kotlin/kotlinx.serialization#android.
- Delete empty config files.

Author: SimonMarquis

app-nia-catalog/proguard-rules.pro

@@ -1,30 +1,3 @@
-# Keep `Companion` object fields of serializable classes.
-# This avoids serializer lookup through `getDeclaredClasses` as done for named companion objects.
--if @kotlinx.serialization.Serializable class **
--keepclassmembers class <1> {
-    static <1>$Companion Companion;
-}
-
-# Keep `serializer()` on companion objects (both default and named) of serializable classes.
--if @kotlinx.serialization.Serializable class ** {
-    static **$* *;
-}
--keepclassmembers class <2>$<3> {
-    kotlinx.serialization.KSerializer serializer(...);
-}
-
-# Keep `INSTANCE.serializer()` of serializable objects.
--if @kotlinx.serialization.Serializable class ** {
-    public static ** INSTANCE;
-}
--keepclassmembers class <1> {
-    public static <1> INSTANCE;
-    kotlinx.serialization.KSerializer serializer(...);
-}
-
-# @Serializable and @Polymorphic are used at runtime for polymorphic serialization.
--keepattributes RuntimeVisibleAnnotations,AnnotationDefault
-
 -dontwarn org.bouncycastle.jsse.BCSSLParameters
 -dontwarn org.bouncycastle.jsse.BCSSLSocket
 -dontwarn org.bouncycastle.jsse.provider.BouncyCastleJsseProvider