Remove @JvmStatic for JDK1.6 compatibility

mirai-core/src/commonMain/kotlin/net.mamoe.mirai/contact/Group.kt

@@ -16,7 +16,6 @@ import net.mamoe.mirai.data.MemberInfo
 import net.mamoe.mirai.event.events.*
 import net.mamoe.mirai.utils.MiraiExperimentalAPI
 import kotlin.jvm.JvmName
-import kotlin.jvm.JvmStatic
 
 /**
  * 群. 在 QQ Android 中叫做 "Troop"
@@ -145,8 +144,7 @@ interface Group : Contact, CoroutineScope {
 
         /**
          * by @kar98k
-         */
-        @JvmStatic
+         */ // don't @JvmStatic: JDK 1.8 required
         fun calculateGroupUinByGroupCode(groupCode: Long): Long {
             var left: Long = groupCode / 1000000L
 
@@ -163,7 +161,6 @@ interface Group : Contact, CoroutineScope {
             return left * 1000000L + groupCode % 1000000L
         }
 
-        @JvmStatic
         fun calculateGroupCodeByGroupUin(groupUin: Long): Long {
             var left: Long = groupUin / 1000000L