qmlls: don't persist documentation DB on Disk

It seems that the code that takes care of loading the documentation files actually persists the documentation DB on disk. If the DB is already in the disk, then qmlls ignores the documentation path it gets and just loads the DB instead. This leads to some weird scenarios:

• qmlls will show documentation hints even when run without --doc-dir because it loads the previous DB when existing
• qmlls will show documentation hints of the first --doc-dir it was invoked with, so multiple qmlls running on different qt versions all show the same documentation hints from the same qt version