drop XC16x bits

Commit 04f096fb ("Move the xc16x target to the obsolete list") moved
the architecture from the "obsolete but still available" to the
"obsolete / support removed" list in config.bfd, making the architecture
impossible to enable (except maybe via "enable everything" options").

Note that I didn't touch */po/*.po{,t} on the assumption that these
would be updated by some (half)automatic means.