scmi: Remove unused SCMI_VERSION

The definition of SCMI_VERSION is not used anywhere in the codebase,
hence it can be removed.
Each SCMI protocol implementation provides a protocol version, i.e. for
power domain is SCMI_PROTOCOL_VERSION_POWER_DOMAIN.

Change-Id: Ie98a7f586ac4875fdd9e802709f2c21b90b3288b
Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>