Revert indexed and enlargable binary heap implementation.

This reverts commit b840508644 and bcb14f4abc. These commits were made
for commit 5bec1d6bc5 (Improve eviction algorithm in ReorderBuffer
using max-heap for many subtransactions). However, per discussion,
commit efb8acc0d0 replaced binary heap + index with pairing heap, and
made these commits unnecessary.

Reported-by: Jeff Davis
Discussion: https://postgr.es/m/12747c15811d94efcc5cda72d6b35c80d7bf3443.camel%40j-davis.com

src/backend/executor/nodeGatherMerge.c

@@ -422,7 +422,6 @@ gather_merge_setup(GatherMergeState *gm_state)
 	/* Allocate the resources for the merge */
 	gm_state->gm_heap = binaryheap_allocate(nreaders + 1,
 											heap_compare_slots,
-											false,
 											gm_state);
 }