This method was introduced to improve the data redistribution
performance during gpexpand phase2, however per benchmark results the
effect does not reach our expectation.  For example when expanding a
table from 7 segments to 8 segments the reshuffle method is only 30%
faster than the traditional CTAS method, when expanding from 4 to 8
segments reshuffle is even 10% slower than CTAS.  When there are indexes
on the table the reshuffle performance can be worse, and extra VACUUM is
needed to actually free the disk space.  According to our experiments
the bottleneck of reshuffle method is on the tuple deletion operation,
it is much slower than the insertion operation used by CTAS.

The reshuffle method does have some benefits, it requires less extra
disk space, it also requires less network bandwidth (similar to CTAS
method with the new JCH reduce method, but less than CTAS + MOD).  And
it can be faster in some cases, however as we can not automatically
determine when it is faster it is not easy to get benefit from it in
practice.

On the other side the reshuffle method is less tested, it is possible to
have bugs in corner cases, so it is not production ready yet.

In such a case we decided to retire it entirely for now, we might add it
back in the future if we can get rid of the slow deletion or find out
reliable ways to automatically choose between reshuffle and ctas
methods.

Discussion: https://groups.google.com/a/greenplum.org/d/msg/gpdb-dev/8xknWag-SkI/5OsIhZWdDgAJReviewed-by: NHeikki Linnakangas <hlinnakangas@pivotal.io>
Reviewed-by: NAshwin Agrawal <aagrawal@pivotal.io>