Remove useless inline marker.

Putting "inline" on a function that's not used anywhere in its
own file is useless unless the linker is doing global optimization,
a method we don't generally enable.  Moreover, it draws warnings
from some buildfarm members (curculio at least).

Looks like this was sloppiness in cc8b25712, which moved the
function from somewhere else where the inline marker was
more appropriate.

src/backend/commands/vacuum.c

@@ -2317,7 +2317,7 @@ vac_cleanup_one_index(IndexVacuumInfo *ivinfo, IndexBulkDeleteResult *istat)
  * Returns the total required space for VACUUM's dead_items array given a
  * max_items value.
  */
-inline Size
+Size
 vac_max_items_to_alloc_size(int max_items)
 {
 	Assert(max_items <= MAXDEADITEMS(MaxAllocSize));