Function 'fraction_free_LU' was declared twice

This was triggering a compiler warning:

.../csympy/src/matrix.h:118:23: warning: ‘void CSymPy::fraction_free_LU(const CSymPy::DenseMatrix&, CSymPy::DenseMatrix&, CSymPy::DenseMatrix&)’ is already a friend of class ‘CSymPy::DenseMatrix’ [enabled by default]

So we just delete the duplicate declaration.

/cc @thilina