pgsql: Fix some const qualifier use in ri_triggers.c

Fix some const qualifier use in ri_triggers.c

The ri_FetchConstraintInfo() and ri_LoadConstraintInfo() functions
were declared to return const RI_ConstraintInfo *, but callers
sometimes need to modify the struct, requiring casts to drop the
const. Remove the misapplied const qualifiers and the casts that
worked around them.

Reported-by: Peter Eisentraut <peter(at)eisentraut(dot)org>
Author: Peter Eisentraut <peter(at)eisentraut(dot)org>
Discussion: https://postgr.es/m/548600ed-8bbb-4e50-8fc3-65091b122276@eisentraut.org

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/1b9dc2cb75745329fc133953a6e86f17ff472d36

Modified Files
--------------
src/backend/utils/adt/ri_triggers.c | 34 ++++++++++++++++------------------
1 file changed, 16 insertions(+), 18 deletions(-)