android: don't use proguard on debug builds

It's not necessary and makes the build faster.

android/app/build.gradle

@@ -34,8 +34,6 @@ android {
 
     buildTypes {
         debug {
-            minifyEnabled true
-            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules-debug.pro'
             buildConfigField "boolean", "GOOGLE_SERVICES_ENABLED", "${googleServicesEnabled}"
             buildConfigField "boolean", "LIBRE_BUILD", "${rootProject.ext.libreBuild}"
         }

android/app/proguard-rules-debug.pro

@@ -1,5 +0,0 @@
--include proguard-rules.pro
-
-# Disabling obfuscation is useful if you collect stack traces from production crashes
-# (unless you are using a system that supports de-obfuscate the stack traces).
--dontobfuscate