[FAB-9783] Delete unused file install_instantiate.rst

Nothing in the docs directory refers to this file in any way,
so it should be safe to get rid of it.

Change-Id: I02dfb276407e51b11d49c7bc5bbfe64dbf850700
Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>

Author: rpjday