Get rid of ojrelid local variable in remove_rel_from_query()

Get rid of ojrelid local variable in remove_rel_from_query()

As spotted by Coverity, the calculation of ojrelid mixes signed and unsigned
types causes possible overflow and undefined behavior.  Instead of trying to
fix the expression, this commit eliminates the relied local variable.  The
explicit branching is used to replace the -1 value.  That, in turn, requires
changing the signature of the remove_rel_from_eclass() function.

Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/914330.1740330169%40sss.pgh.pa.us
Reviewed-by: Andrei Lepikhov <lepihov@gmail.com>