Drop undefined B macro from tar man page, (fixes lintian warning)

This undefined macro was causing misformatting of the first line
of the man page:

   The first argument to should be...

rather than:

   The first argument to tar should be...

Presumably just dropping it misses out on some intending formatting,
(emboldening of the word tar perhaps?), but this is at least better
than the word being missing.