Don't explicitly build and link rx_pthread.o

Don't explicitly build and link rx_pthread.o

A number of pthreaded directories explicitly build, and link, with
rx_pthread.o, in order to be able to send RX 'dpf' debugging messages
to the server log, rather than to stderr.

However, this direct linking causes build failures on some platforms,
because we end up linking in a version of rx_pthread.o, twice and both
versions contain identical linker symbols.

As this functionality is only used for debugging, just get rid of the
additional object.

Change-Id: I6126891db2dcbc289f6a9f7b99c01d6af7a19089
Reviewed-on: http://gerrit.openafs.org/8071
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>