psql: Add tab completion for DELETE ... USING.

psql: Add tab completion for DELETE ... USING.

This implements the tab completion that was marked as XXX TODO in the
source code. The following completion is now supported:

    DELETE FROM <table> USING <TAB> -> list of relations supporting SELECT

This uses Query_for_list_of_selectables (instead of Query_for_list_of_tables)
because the USING clause can reference not only tables but also views and
other selectable objects, following the same syntax as the FROM clause
of a SELECT statement.

Author: Tatsuya Kawata <kawatatatsuya0913@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Kirill Reshke <reshkekirill@gmail.com>
Reviewed-by: Soumya S Murali <soumyamurali.work@gmail.com>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/CAHza6qf0CLJuJr+5cQw0oWNebM5VyMB-ghoKBgnEjOQ_JtAiuw@mail.gmail.com