objectcache: Remove unused key ($i) in foreach() loop

In I6b3167cac824d8bd8773bc66c386f41e4d380021 (75aec3686a2e3f4d0),
awesome Krinkle did some huge refactor of the BagOStuff parent and
subclasses to reduce boiler plate and indirection logic around the
makeKey() method.

In the process of doing that, the key that was previously used in
the foreach() loop in the makeKeyInternal() method was no longer
used.

The main intention of the $i was to append a colon(:) delimiter
infront of the keyspace before concatenating the various components
values to construct a cache key for the given keyspace and components.

Now that it's been handled differently, it's time to say farewell
to the $i.

Change-Id: I03a4b85509a9b841b05d000afb6242f91e06d9be