delete the Greek `extended` variant, as it does not extend `basic` at all

Until two years ago, the extended variant contained fewer symbols than the basic variant, and did not contain any symbols that basic did not have. The extra symbols that it had over the simple variant were all present in the same locations in the basic variant. So, the extended variant was fully redundant.

This addresses and closes issue #412 (closed).

Signed-off-by: Benno Schulenberg bensberg@telfort.nl