[fix] makefile.sphinx: additions to commit c2a6f145

In commit c2a6f145 the sphinx-doc and sphinx-live targets has been removed,
since Sphinx is already installed by the pyenvinstall target.

This commit removes the targets from the remaining prerequisites of the 'BOOK'
targets

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>