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/MDC/MDCLoader.cpp

@@ -110,19 +110,9 @@ MDCImporter::~MDCImporter() {
 
 // ------------------------------------------------------------------------------------------------
 // Returns whether the class can handle the format of the given file.
-bool MDCImporter::CanRead(const std::string &pFile, IOSystem *pIOHandler, bool checkSig) const {
-    const std::string extension = GetExtension(pFile);
-    if (extension == "mdc") {
-        return true;
-    }
-
-    // if check for extension is not enough, check for the magic tokens
-    if (!extension.length() || checkSig) {
-        uint32_t tokens[1];
-        tokens[0] = AI_MDC_MAGIC_NUMBER_LE;
-        return CheckMagicToken(pIOHandler, pFile, tokens, 1);
-    }
-    return false;
+bool MDCImporter::CanRead(const std::string &pFile, IOSystem *pIOHandler, bool /*checkSig*/) const {
+    static const uint32_t tokens[] = { AI_MDC_MAGIC_NUMBER_LE };
+    return CheckMagicToken(pIOHandler, pFile, tokens, std::size(tokens));
 }
 
 // ------------------------------------------------------------------------------------------------