Remove various unnecessary (char *) casts

Remove a number of (char *) casts that are unnecessary.  Or in some
cases, rewrite the code to make the purpose of the cast clearer.

Reviewed-by: Dagfinn Ilmari Mannsåker <ilmari@ilmari.org>
Discussion: https://www.postgresql.org/message-id/flat/fd1fcedb-3492-4fc8-9e3e-74b97f2db6c7%40eisentraut.org

src/backend/storage/page/bufpage.c

@@ -1502,7 +1502,7 @@ PageSetChecksumCopy(Page page, BlockNumber blkno)
 
 	/* If we don't need a checksum, just return the passed-in data */
 	if (PageIsNew(page) || !DataChecksumsEnabled())
-		return (char *) page;
+		return page;
 
 	/*
 	 * We allocate the copy space once and use it over on each subsequent