Remove the .pyf interface definition

Do not remove the wignerSymbols.f source file as this will still
be useful for the C/Fortran bindings - see #14

src/fortran/wignerSymbols.pyf

-!    -*- f90 -*-
-
-! SPDX-FileCopyrightText: 2018 Christopher Kerr
-!
-! SPDX-License-Identifier: CC0-1.0
-
-python module wignerSymbols
-    interface
-        subroutine shzmat(LMAX, DLMKP)
-            integer, intent(in) :: LMAX
-            double precision, intent(out), depend(LMAX), dimension((LMAX+1)*(LMAX+2)*(LMAX+2)*(LMAX+3)/12) :: DLMKP
-        end subroutine shzmat
-    end interface
-end python module wignerSymbols