Fix inconsistencies and typos in the tree

This is numbered take 7, and addresses a set of issues around:
- Fixes for typos and incorrect reference names.
- Removal of unneeded comments.
- Removal of unreferenced functions and structures.
- Fixes regarding variable name consistency.

Author: Alexander Lakhin
Discussion: https://postgr.es/m/10bfd4ac-3e7c-40ab-2b2e-355ed15495e8@gmail.com

src/backend/access/gist/README

@@ -170,7 +170,7 @@ it splits the page, and constructs the new downlink tuples for the split
 pages. The caller must then call gistplacetopage() on the parent page to
 insert the downlink tuples. The parent page that holds the downlink to
 the child might have migrated as a result of concurrent splits of the
-parent, gistfindCorrectParent() is used to find the parent page.
+parent, gistFindCorrectParent() is used to find the parent page.
 
 Splitting the root page works slightly differently. At root split,
 gistplacetopage() allocates the new child pages and replaces the old root

src/backend/access/gist/gist.c

@@ -821,7 +821,7 @@ gistdoinsert(Relation r, IndexTuple itup, Size freespace,
 			/*
 			 * Leaf page. Insert the new key. We've already updated all the
 			 * parents on the way down, but we might have to split the page if
-			 * it doesn't fit. gistinserthere() will take care of that.
+			 * it doesn't fit. gistinserttuple() will take care of that.
 			 */
 
 			/*