Remove useless AM_CONDITIONAL STARPU_USE_PAPI
While using StarPU's configure.ac as an example to use autotools in another project, I noticed this
STARPU_USE_PAPI is used nowhere in the project, so I think we can delete this line from the configure.ac.
Since I'm not completely sure about this change (maybe this line is really necessary !) I make a MR rather than directly committing to master.