afs: retire HAVE_LINUX_COMPLETION_H conditionals
Now that support for linux 2.4 has been sunset, as of commit
ccf353ede6ef5cce7c562993d1bea0d20844bdb7, it is no longer necessary to
put conditional compilation checks around the linux wait-for-completion
functions, which were introduced sometime during the linux 2.4 series
and have been available since.
Also, remove the remnant LINUX_COMPLETION_H_EXISTS autoconf macro, which
was removed from use in commit
ef8bd5a29b937a1211540aa60398ee966470a712.
Change-Id: Iea974236f73eef8c567a897d6a473254edf95379
Reviewed-on: https://gerrit.openafs.org/12278
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>