The version is maintained in libchrome-version.eclass file now.
It should be the source of truth, and we should use the variable passed from here for any build.

Following will be removed;
- version suffix from metrics and libbrillo build rules.
- common-mk/BASE_VER.
to simplify the libchrome uprev procedure.

Specifically, common-mk/BASE_VER is used as default value if BASE_VER env var is not set. With the removal of the file, in such a case, the build should fail simply.