[identity] Remove identity prefix from token store and cache.

Following earlier discussions with Satsuki the precedent we set with
these packages didn't make sense when applied to token manager. We're
probably stomping on a wider namespace than we'd ideally like but thats
a common theme across rust package names at the moment plus its
possible we'll roll both these into token manager at some point anyway.

Change-Id: I825ca9e0a853cdafde845e96077a64d1d5729bc4