doc: remove 1.5 doc link until after release

Link to the 1.5 version of documentation shouldn't be added
to the documentation home page until AFTER the release documents
are pushed to the site.  (I thought this would have been done when
the code freeze happened.)  Remove the 1.5 link for now and wait
until after the 1.5 docs are pushed.

Jira: ZEP-730
Change-Id: Ia225c9ce3ffb52540fb43f105a3cd40e72d0cf4a
Signed-off-by: David B. Kinder <david.b.kinder@intel.com>