core: remove PluginManager

mirai-console/build.gradle

@@ -15,4 +15,4 @@ sourceCompatibility = "11"
 
 tasks.withType(JavaCompile) {
     options.encoding = "UTF-8"
-}
+}
\ No newline at end of file

mirai-core/src/jvmMain/kotlin/net/mamoe/mirai/plugin/MiraiPluginManager.java

-package net.mamoe.mirai.plugin;
-
-/**
- * @author NaturalHG
- */
-public class MiraiPluginManager {
-
-
-}