decompressionwriter: remove check for set _dctx
decompressionwriter: remove check for set _dctx

Now that we call _ensure_dctx() as part of __init__, this is
no longer necessary.

The C code didn't have this restriction for whatever reason.

indygreg/python-zstandard,indygreg/python-zstandard,indygreg/python-zstandard,indygreg/python-zstandard