gh-39092: Remove erroneous member declaration in farey_symbol
    
As you can see in the C++ header file, there is in fact no members named
`a, b, c, d`. In fact it's an error in C++ to have a member function and
member variable with the same name.

Newer versions of Cython gives hard-to-debug errors for this.

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URL: #39092
Reported by: user202729
Reviewer(s): Julian Rüth

Author: Release Manager

src/sage/modular/arithgroup/farey_symbol.pyx

@@ -47,7 +47,6 @@ from sage.structure.richcmp cimport richcmp_not_equal
 
 cdef extern from "sl2z.hpp":
     cppclass cpp_SL2Z "SL2Z":
-        mpz_class a, b, c, d
         cpp_SL2Z(int, int, int, int)
         cpp_SL2Z(mpz_class, mpz_class, mpz_class, mpz_class)
         mpz_class a()