Revert "assume scheme files are chicken"
Revert "assume scheme files are chicken"

This reverts commit dfabe63c62d4238f859be851734598102915a3cf.

- because
  - docs indicate issues remain with vim chicken support regardless of
    specifying `b:is_chicken=1`
    - see
      - https://wiki.call-cc.org/vim#coloring

atweiden/pacfiles,atweiden/pacfiles