Remove the usage of SystemProperties.set()

The usage of SystemProperties.set() in MockableSystemProperties
is either replaced by sysprop or removed. Thus, remove it to
suppport incoming ConnectivityService mainline.

Bug: 170917042
Test: m ; atest FrameworksNetTests
Change-Id: I99b3f200985c43e9338d828bd5b6bee323d61579

services/core/java/com/android/server/connectivity/MockableSystemProperties.java

@@ -38,9 +38,4 @@ public class MockableSystemProperties {
     public void setTcpInitRwnd(int value) {
         NetworkProperties.tcp_init_rwnd(value);
     }
-    // TODO(b/33308258): SystemProperties.set() will not be accessible if ConnectivityService
-    // becomes a mainline module. Remove it and replace the usage after removing net.dns*.
-    public void set(String key, String value) {
-        SystemProperties.set(key, value);
-    }
 }