Simplified importer search and fixed a few bugs

The search for a matching importer had a few issues, see #3791. There were two different mechanisms to determine whether an importer accepts a specific file extension:
1. `aiImporterDesc::mFileExtensions`, which was forwarded to the UI via `BaseImporter::GetExtensionList()`.
2. `BaseImporter::CanRead()` when called with `checkSig == false`, which determines whether to actually use that importer.
Both were redundant and got out of sync repeatedly. I removed 2. completely and replaced it with 1., thereby syncing UI/import and shortening all `BaseImporter::CanRead()` implementations.

Further bugfixes:
- fixed glTF2 importer throwing exceptions when checking whether it can load a file
- removed `BaseImporter::SimpleExtensionCheck()` because it is no longer used and had a bug with case sensitivity

Since the `checkSig` parameter in `BaseImporter::CanRead()` is now useless, it can be removed completely. I’m not sure if this would break ABI compatiblity, so I’ll submit it with a later pull request.

code/AssetLib/Obj/ObjFileImporter.cpp

@@ -87,16 +87,10 @@ ObjFileImporter::~ObjFileImporter() {
 }
 
 // ------------------------------------------------------------------------------------------------
-//  Returns true, if file is an obj file.
-bool ObjFileImporter::CanRead(const std::string &pFile, IOSystem *pIOHandler, bool checkSig) const {
-    if (!checkSig) {
-        //Check File Extension
-        return SimpleExtensionCheck(pFile, "obj");
-    } else {
-        // Check file Header
-        static const char *pTokens[] = { "mtllib", "usemtl", "v ", "vt ", "vn ", "o ", "g ", "s ", "f " };
-        return BaseImporter::SearchFileHeaderForToken(pIOHandler, pFile, pTokens, 9, 200, false, true);
-    }
+//  Returns true if file is an obj file.
+bool ObjFileImporter::CanRead(const std::string &pFile, IOSystem *pIOHandler, bool /*checkSig*/) const {
+    static const char *tokens[] = { "mtllib", "usemtl", "v ", "vt ", "vn ", "o ", "g ", "s ", "f " };
+    return BaseImporter::SearchFileHeaderForToken(pIOHandler, pFile, tokens, std::size(tokens), 200, false, true);
 }
 
 // ------------------------------------------------------------------------------------------------