refactor(ui): 移除废弃的技能UI预制件并调整布局

1. 删除了mskills.prefab及其元数据文件
2. 调整了mission.prefab中三处文本的实际字体大小从31改为25
3. 重新排布了MissSkillsComp的技能槽位置

assets/resources/gui/element/mission.prefab

@@ -16174,7 +16174,7 @@
     "_string": "名称",
     "_horizontalAlign": 1,
     "_verticalAlign": 1,
-    "_actualFontSize": 31,
+    "_actualFontSize": 25,
     "_fontSize": 30,
     "_fontFamily": "Arial",
     "_lineHeight": 35,
@@ -20131,7 +20131,7 @@
     "_string": "名称",
     "_horizontalAlign": 1,
     "_verticalAlign": 1,
-    "_actualFontSize": 31,
+    "_actualFontSize": 25,
     "_fontSize": 30,
     "_fontFamily": "Arial",
     "_lineHeight": 35,
@@ -24085,7 +24085,7 @@
     "_string": "名称",
     "_horizontalAlign": 1,
     "_verticalAlign": 1,
-    "_actualFontSize": 31,
+    "_actualFontSize": 25,
     "_fontSize": 30,
     "_fontFamily": "Arial",
     "_lineHeight": 35,