Remove wrongly backpatched piece of code in cube.c

Due to sloppy division of changes between f50c80db (which was not
back-patched) and 563a053b, this piece of code was wrongly backpatched to
REL_10_STABLE and REL9_6_STABLE.  This code never causes real error because
its condition is never satisfied, but it's a dead code, which needs to be
removed.

Alexander Korotkov per gripe from Tom Lane