mpi: remove unused i586 and pentium4 assembly

* mpi/config.links: Remove 'i586' from paths.
* mpi/i586*: Remove.
* mpi/pentium4/*: Remove.

Current x86 targets (i686) have been defaulting on mpi/i386 assembly
for quite some time now. Remove mpi/i586 as it is no longer used. While
at it, remove mpi/pentium4 assembly also as obsolete.

• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>