remove second grdecl.h header file

that one was completely unused and the "grdecl" struct defined in
there collided name-wise with the one defined in "GridManager.hpp".
trying to link code which used the version of the grdecl.h header with
code that used the one from GridManager.hpp would have resulted in _a
lot_ hilarity...
Merge pull request #568 from andlaus/remove_grdecl_h

remove second grdecl.h header file