Create a routine PageIndexMultiDelete() that replaces a loop around

PageIndexTupleDelete() with a single pass of compactification ---
logic mostly lifted from PageRepairFragmentation.  I noticed while
profiling that a VACUUM that's cleaning up a whole lot of deleted
tuples would spend as much as a third of its CPU time in
PageIndexTupleDelete; not too surprising considering the loop method
was roughly O(N^2) in the number of tuples involved.