Remove no-longer-used AdminServletPathPrefix config value

We still have some paths under /_dr/admin (e.g. for server-side regtool
actions), but this actual config value isn't used anywhere ever since
we deleted the admin console in []
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=119693711

java/com/google/domain/registry/config/RegistryConfig.java

@@ -183,11 +183,6 @@ public interface RegistryConfig {
    */
   public ImmutableList<String> getRegistrarChangesNotificationEmailAddresses();
 
-  /**
-   * Returns the relativeURL path to the admin servlet, e.g. "/_dr/admin".
-   */
-  public String getAdminServletPathPrefix();
-
   /**
    * Returns default WHOIS server to use when {@code Registrar#getWhoisServer()} is {@code null}.
    *