[script] Remove useless assignments

- If FRAMAC_SHARE is not already set, either frama-c -print-share-path
  gives a correct result, and there's no need to set FRAMAC_SHARE in the
  first place, or it does not, and putting this result in FRAMAC_SHARE
  won't do any good.
- there's no need (and it seems to confuse Frama-C) to -add-path when in
  internal mode.