Remove shared_empty and unsharable_empty from API

Remove shared_empty and unsharable_empty from API

They still exist and help avoid allocation of "empty" array headers, but
they're no longer part of the public API, thus reducing relocatable
symbols and relocations in inline code.

This means an extra non-inline call on QArrayDataPointer::clear and
setSharable operations, which are (expensive) detaching operations,
anyway.

Change-Id: Iea804e5ddc8af55ebc0951ca17a7a4e8401abc55
Reviewed-by: Bradley T. Hughes <bradley.hughes@nokia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>