Remove resources folders in subdirectories (#8327)

The `resources` directories in `flutter-idea` and `flutter-studio` were
symlinks to the main-level directory, and I think this isn't needed with
a flat structure. The classes in the generated `flutter-intellij.jar`
file seem to include the same ones as the old jar and the plugin
installs correctly.

Author: helin24

build.gradle.kts

@@ -300,7 +300,7 @@ sourceSets {
     resources.srcDirs(
       listOf(
         "flutter-idea/src",
-        "flutter-idea/resources"
+        "resources"
       )
     )
     java.srcDirs(
@@ -320,7 +320,7 @@ sourceSets {
     )
     resources.srcDirs(
       listOf(
-        "flutter-idea/resources",
+        "resources",
         "flutter-idea/testData",
         "flutter-idea/testSrc/unit"
       )