Delete proguard

As discussed in https://github.com/nextcloud/android-library/issues/160 proguard.cfg is not correctly working.
As we do not use it, this should be removed.

proguard.cfg

@@ -1,38 +0,0 @@
--optimizationpasses 5
--dontusemixedcaseclassnames
--dontskipnonpubliclibraryclasses
--dontpreverify
--verbose
--optimizations !code/simplification/arithmetic,!field/*,!class/merging/*
-
--keep public class * extends android.app.Activity
--keep public class * extends android.app.Application
--keep public class * extends android.app.Service
--keep public class * extends android.content.BroadcastReceiver
--keep public class * extends android.content.ContentProvider
--keep public class * extends android.app.backup.BackupAgentHelper
--keep public class * extends android.preference.Preference
--keep public class com.android.vending.licensing.ILicensingService
-
--keepclasseswithmembernames class * {
-    native <methods>;
-}
-
--keepclasseswithmembers class * {
-    public <init>(android.content.Context, android.util.AttributeSet);
-}
-
--keepclasseswithmembers class * {
-    public <init>(android.content.Context, android.util.AttributeSet, int);
-}
-
--keepclassmembers enum * {
-    public static **[] values();
-    public static ** valueOf(java.lang.String);
-}
-
--keep class * implements android.os.Parcelable {
-  public static final android.os.Parcelable$Creator *;
-}
-
--keepattributes InnerClasses