Some of the plantuml diagrams in the I/O storage abstraction layer
documentation are absent from the rendered version of the porting
guide. The build log (see [1] for example) reports a syntax error in
these files. This is due to the usage of the 'order' keyword on the
participants list, which does not seem to be supported by the version
of plantuml installed on the ReadTheDocs server.

Fix these syntax errors by removing the 'order' keyword altogether. We
simply rely on the participants being declared in the desired order,
which will be the order of display, according to the plantuml
documentation.

[1] https://readthedocs.org/api/v2/build/9870345.txt

Change-Id: Ife35c74cb2f1dac28bda07df395244639a8d6a2b
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>