Remove plugin search feature (#14759)

Co-authored-by: SUZUKI Sosuke <aosukeke@gmail.com>

tests/integration/cli/config/plugin-search-dirs/.prettierrc

@@ -1,3 +1,2 @@
 {
-    "pluginSearchDirs": false
 }

tests/integration/cli/config/resolve-relative/.prettierrc

@@ -1,4 +1,3 @@
 {
-    "plugins": ["../path-to-plugin"],
-    "pluginSearchDirs": ["../path-to-plugin-search-dir"]
+    "plugins": ["../path-to-plugin"]
 }