OPENNLP-1661: Fix custom models being wiped from OpenNLP user.home directory

[mawiesne]

Change

• deletes AbstractDownloadUtilTest.java removing historical code that wiped models (accidentally)
• adds package-private DownloadUtil#existsModel(..) method to check if models of certain language exist locally
• adds to and adjusts related test classes

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[rzo1]

Left some comments, mainly about control flow by exception for NoSuchFileException in exists(...) to avoid opening a connection to get the sha512 hash.