support ALTER TABLE DROP EXPRESSION for virtual generated column

hi.

the attached patch is to implement $subject.
the generation expression will be dropped.
the column value previous was NULL will be materialized
based on generation expression.

It seems fairly straightforward:
drop the generation expression in ATExecDropExpression,
and instruct phase 3 to compute the generation expression
and do the table rewrite.

the doc changes:

<para>
- This form turns a stored generated column into a normal base column.
- Existing data in the columns is retained, but future changes will no
- longer apply the generation expression.
- </para>
-
- <para>
- This form is currently only supported for stored generated columns (not
- virtual ones).
+ This form turns a generated column into a normal base column.
+ For stored generated column, existing data in the columns is retained;
+ For virtual generated column, it will compute the generation
expression and
+ store the value in the columns. For inheritance hierarchy or
partition hierarchy,
+ the virtual generation expression is computed based on the
child's own generation expression.
+ The future changes will no longer apply the generation expression.
</para>