refac(shallow_t):rm unnecessary defined assignment

Author: rouson

test/shallow_m.f90

@@ -8,8 +8,6 @@ module shallow_m
     type, extends(ref_reference_t) :: shallow_t
         integer, pointer :: ref => null()
     contains
-        procedure :: shallow_copy
-        generic :: assignment(=) => shallow_copy
         procedure :: free
     end type
 
@@ -37,10 +35,4 @@ subroutine free(self)
         resource_freed = .true.
     end subroutine
 
-    subroutine shallow_copy(lhs, rhs)
-       class(shallow_t), intent(inout) :: lhs
-       type(shallow_t), intent(in) :: rhs
-       lhs%ref => rhs%ref
-    end subroutine
-
 end module