Rename several destroy* functions/tags to drop*.

src/backend/access/gist/gist.c

src/backend/access/hash/hash.c

src/backend/access/nbtree/nbtree.c

src/backend/access/rtree/rtree.c

src/backend/access/transam/xact.c

src/backend/catalog/heap.c

src/backend/catalog/index.c

src/backend/commands/cluster.c

src/backend/commands/command.c

src/backend/commands/creatinh.c

src/backend/commands/dbcommands.c

src/backend/commands/indexcmds.c

src/backend/commands/remove.c

src/backend/commands/vacuum.c

src/backend/commands/view.c

src/backend/executor/_deadcode/nodeTee.c

src/backend/executor/execMain.c

src/backend/executor/execTuples.c

src/backend/executor/nodeHash.c

src/backend/executor/nodeMaterial.c