consider abandoing locking in FileHistoryCache#writeObjectToFile()

[vladak]

There is a curious comment about locking in

opengrok/opengrok-indexer/src/main/java/org/opengrok/indexer/history/FileHistoryCache.java

Lines 209 to 224 in d2e8d64

	// We have a problem that multiple threads may access the cache layer
	// at the same time. Since I would like to avoid read-locking, I just
	// serialize the write access to the cache file. The generation of the
	// cache file would most likely be executed during index generation, and
	// that happens sequentially anyway....
	// Generate the file with a temporary name and move it into place when
	// done, so it is not necessary to protect the readers for partially updated
	// files...
	final File output;
	try {
	output = File.createTempFile("ogtmp", null, dir);
	CacheUtil.writeCache(object, output);
	} catch (IOException ioe) {
	throw new HistoryException("Failed to write history", ioe);
	}
	synchronized (lock) {

I think this pertains to the times when the history cache could be generated on the fly, however this is no longer the case (#3828), so I think this lock can be removed.