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

(cherry picked from commit 13d5524a0dec75df4d80f3fca4f266cb790f1b76)