Doc: remove claim that hash index creation depends on effective_cache_size.

This text was added by commit ff213239, and not long thereafter obsoleted
by commit 4adc2f72 (which made the test depend on NBuffers instead); but
nobody noticed the need for an update.  Commit 9563d5b5 adds some further
dependency on maintenance_work_mem, but the existing verbiage seems to
cover that with about as much precision as we really want here.  Let's
just take it all out rather than leaving ourselves open to more errors of
omission in future.  (That solution makes this change back-patchable, too.)

Noted by Peter Geoghegan.

Discussion: <CAM3SWZRVANbj9GA9j40fAwheQCZQtSwqTN1GBTVwRrRbmSf7cg@mail.gmail.com>

doc/src/sgml/ref/create_index.sgml

@@ -568,16 +568,7 @@ Indexes:
    dependent on the setting of <xref linkend="guc-maintenance-work-mem">.
    Larger values will reduce the time needed for index creation, so long
    as you don't make it larger than the amount of memory really available,
-   which would drive the machine into swapping.  For hash indexes, the
-   value of <xref linkend="guc-effective-cache-size"> is also relevant to
-   index creation time: <productname>PostgreSQL</productname> will use one
-   of two different hash index creation methods depending on whether the
-   estimated index size is more or less than <varname>effective_cache_size</>.
-   For best results, make sure that this parameter is also set to something
-   reflective of available memory, and be careful that the sum of
-   <varname>maintenance_work_mem</> and <varname>effective_cache_size</> is
-   less than the machine's RAM less whatever space is needed by other
-   programs.
+   which would drive the machine into swapping.
   </para>
 
   <para>