* Remove QTextCodec
QTextCodec was removed from Qt6 and is only available through the
Qt5Compat module.
QTextCodec was only used by the HydrogenImport plugin when importing old
Hydrogen files that were saved using TinyXML before it supported UTF-8.
HydrogenImport would use QTextCodec to try to get the current encoding
from the locale, and then use that as a best guess for interpreting the
XML data in the unspecified encoding it was saved in. None of this was
ever reliable, since the encoding of the computer that saved the
Hydrogen file might not be the same as the computer running LMMS and
importing that file.
There is no good solution here, so I decided to simply assume the old
Hydrogen files are UTF-8 encoded. The worst that can happen is someone's
ancient Hydrogen files containing non-ASCII text of some random encoding
becomes mojibake'd after importing into LMMS, which is something that
already could have happened.
* Clean up a little
