HPCC-11852 Remove calls to getDatasetName from indexwrite

Going to some lengths to deduce a value that is never used...

I will look at removing the generation of the getDatasetname helpr in a
subsequent commit (there may be compatibility reasons we don't want to do
that).

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
HPCC-11852 Remove calls to getDatasetName from indexwrite

Prevent generation of this virtual, and mark it as deprecated in the VMT and
the grammar.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #6242 from richardkchapman/getdatasetname

HPCC-11852 Remove calls to getDatasetName from indexwrite

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>