sim_fibo_ad: remove compatibility code for old parser

this makes the simulator quite a bit more maintainable: setting
USE_NEW_PARSER to 0 did not even compile after the the constructor for
the wells manager which took the old deck was removed last
week. Since, according to Atgeirr, SPE-1 is now producing exactly the
same results as before, it also does no longer make too much sense to
keep that code on life support...

(this is an unrelated prelude of the small EclipseWriter refactoring I plan on finishing tomorrow, but I stumbled over it...)