afs: Introduce afs_FreeFirstToken

afs: Introduce afs_FreeFirstToken

Change afs_FreeOneToken to unlink the given token from its container,
instead of requiring its caller to do so. Rename the function to
afs_FreeFirstToken, to help indicate the change in behavior.

Also, while we are changing afs_FreeTokens to accommodate this change,
simplify afs_FreeTokens a little, making it resemble
afs_DiscardExpiredTokens a bit more.

[kaduk@mit.edu: add note about dead store elimination]

Change-Id: I0cf9d8b94236c736001a38cccfa7fdfff9f3e609
Reviewed-on: https://gerrit.openafs.org/13807
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: Benjamin Kaduk <kaduk@mit.edu>