graphics/mesa-devel: discard user edits to libmap.conf(5)

- pkg32.sh from emulators/wine* uses RUN_SCRIPTS=false, so
  mesa-devel.conf.sample isn't copied to mesa-devel.conf
- other ports with libmap.conf(5) support don't use @sample
- partially overriding mesa-{dri,libs} is only useful for debugging