doc: Remove version and release variables from conf.py

doc: Remove version and release variables from conf.py

We would need to update this version for the release but, in fact,
it is not required for our publishing workflow; the hosted version
of the docs uses git commit/tag information in place of these
variables anyway.

Instead of updating the version, just remove these variables
entirely.

Change-Id: I424c4e45786e87604e91c7197b7983579afe4806
Signed-off-by: Paul Beesley <paul.beesley@arm.com>