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