man: remove .man_fixup workaround

The whole thing is quite messy - the file is used to indicate that the
man pages were correctly generated prior to applying the "fixup" (alias)

At the same time we use a rule with the same name, to create the same
file if the generation has failed.

In other words - it attempts to create the file either way. So there is
little point in it and we can remove it.

Spotted while attempting to build with bmake which kindly blocked on the
following (non compliant construct)

.man_fixup: | $(miscman_DATA)

Cc: Jonathan Gray <jsg@jsg.id.au>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>