Remove obsolete Retrofit keep rules from the network module

The three rules came from a 2022 comment on Retrofit issue #3751. Retrofit
3.0.0 now ships META-INF/proguard/retrofit2.pro, which contains two of them
verbatim and covers retrofit2.Call through its conditional return-type rule.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

libraries/network/build.gradle.kts

@@ -13,12 +13,6 @@ plugins {
 
 android {
     namespace = "io.element.android.libraries.network"
-
-    buildTypes {
-        release {
-            consumerProguardFiles("consumer-rules.pro")
-        }
-    }
 }
 
 setupDependencyInjection()