Fix Coverity reports of uninitialized scalar variable

Fixes most current reports on Coverity Scan of uninitialized scalar
variable (CWE-457): https://cwe.mitre.org/data/definitions/457.html

These happen most of the time (in our code) when instanciating structs
without a constructor (or with an incomplete one), and later returning
the instance. This is sometimes intended though, as some parameters are
only used in some situations and should not be double-initialized for
performance reasons (e.g. `constant` in ShaderLanguage::Token).

editor/plugins/asset_library_editor_plugin.h

@@ -241,7 +241,6 @@ class EditorAssetLibrary : public PanelContainer {
 
 		bool active;
 		int queue_id;
-		int asset_id;
 		ImageType image_type;
 		int image_index;
 		String image_url;