CdmUtils: remove MUST_EXIST_FILE and getResourceFolderSeperator()
follow up to #8189
The getResourceFolderSeperator() method is not needed at all. The separator character is only used for building file names which are passed to CdmUtils.getUtf8ResourceReader(). This method is internally using the class loader which knows how to deal with different environments and thus expects
/ as separator in the filename.