Remove p_is_insert from struct ParseState.

Remove p_is_insert from struct ParseState.

The only place that used p_is_insert was transformAssignedExpr(),
which used it to distinguish INSERT from UPDATE when handling
indirection on assignment target columns -- see commit c1ca3a19df3.
However, this information is already available to
transformAssignedExpr() via its exprKind parameter, which is always
either EXPR_KIND_INSERT_TARGET or EXPR_KIND_UPDATE_TARGET.

As noted in the commit message for c1ca3a19df3, this use of
p_is_insert isn't particularly pretty, so have transformAssignedExpr()
use the exprKind parameter instead. This then allows p_is_insert to be
removed entirely, which simplifies state management in a few other
places across the parser.

Author: Viktor Holmberg <v@viktorh.net>
Reviewed-by: Dean Rasheed <dean.a.rasheed@gmail.com>
Discussion: https://postgr.es/m/badc3b4c-da73-4000-b8d3-638a6f53a769@Spark