Is it safe to remove 2 "extra" jack studs to make a 60" wide door opening wider?
The use of two jack studs under each end of the header could have been done for any number of reasons. 


If the header supports a lot of weight from above it may have been
deemed desirable to have the extra support on each end.
If the header has an long span it is sometimes desirable to add
additional support for the header.
As you suggested it is possible that the rough opening was
originally made wider than needed for the door that was actually
installed.
The original contractor may have wanted there to be extra wide solid
wall support at each side of the opening than the normal 3" offered
by just one stud + jack stud. It is not at all uncommon to do this
for large window openings in professional construction where it is
known that drapery hardware will mount beyond the width of any
window casings.