[fuchsia] Remove now unnecessary diagnostics directory (flutter/engine#51180)

Inspect is now published using InspectSink and we can get rid of this
directory.

I'm unsure what "debug" and "ctrl" are for so I left them untouched.

Bug: https://g-issues.fuchsia.dev/issues/320785253

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engine/src/flutter/shell/platform/fuchsia/dart_runner/dart_component_controller.cc

@@ -231,7 +231,7 @@ bool DartComponentController::CreateAndBindNamespace() {
       dart_outgoing_dir_ptr_to_check_on_open_.NewRequest());
 
   // Collect our standard set of directories.
-  std::vector<std::string> other_dirs = {"debug", "ctrl", "diagnostics"};
+  std::vector<std::string> other_dirs = {"debug", "ctrl"};
 
   dart_outgoing_dir_ptr_to_check_on_open_.events().OnOpen =
       [this, other_dirs](zx_status_t status, auto unused) {

engine/src/flutter/shell/platform/fuchsia/flutter/component_v2.cc

@@ -274,7 +274,7 @@ ComponentV2::ComponentV2(
 
   // Collect our standard set of directories along with directories that are
   // included in the cml file to expose.
-  std::vector<std::string> other_dirs = {"debug", "ctrl", "diagnostics"};
+  std::vector<std::string> other_dirs = {"debug", "ctrl"};
   for (auto dir : metadata.expose_dirs) {
     other_dirs.push_back(dir);
   }