component/firefox: don't set a default part here

other sections are supposed to use this macros and define a part. Having
a default here is useless and maybe confusing

component/firefox/buildout.cfg

@@ -31,7 +31,6 @@ part = ${firefox:location}
 
 recipe = slapos.recipe.build
 location = ${buildout:bin-directory}/${:wrapper-name}
-part = ${firefox:location}
 script =
   firefox = self.options['part']
   part = self.buildout[os.path.basename(firefox)]