jfs: fix bogus variable self-initialization

A statement was originally added in 2006 to shut up a gcc warning,
now but now clang warns about it:

fs/jfs/jfs_txnmgr.c:1932:15: error: variable 'pxd' is uninitialized when used within its own initialization
      [-Werror,-Wuninitialized]
                pxd_t pxd = pxd;        /* truncated extent of xad */
                      ~~~   ^~~

Modern versions of gcc are fine without the silly assignment, so just
drop it. Tested with gcc-4.6 (released 2011), 4.7, 4.8, and 4.9.

Fixes: c9e3ad60

 ("JFS: Get rid of "may be used uninitialized" warnings")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>