[range.join.view] Remove duplicative condition (#4957)

This function is constrained on `is_reference_v<range_reference_t<const V>>` already,
so checking it again in the `if constexpr` has no effect.

Author: timsong-cpp

source/ranges.tex

@@ -5397,7 +5397,6 @@
     requires @\libconcept{input_range}@<const V> &&
              is_reference_v<range_reference_t<const V>> {
       if constexpr (@\libconcept{forward_range}@<const V> &&
-                    is_reference_v<range_reference_t<const V>> &&
                     @\libconcept{forward_range}@<range_reference_t<const V>> &&
                     @\libconcept{common_range}@<const V> &&
                     @\libconcept{common_range}@<range_reference_t<const V>>)