Fix more warnings like #475.

With clang 12 I get a lot more warnings (or errors, in maintainer mode)
about `[[likely]]` or `[[unlikely]]` in `if constexpr`.  Remove those
attributes.

NEWS

@@ -2,6 +2,7 @@
  - Fix `stream_to` for differing table/client encodings.  (#473)
  - Use `[[likely]]` & `[[unlikely]]` only in C++20, to silence warnings.
  - Fix clang "not a const expression" error in Windows.  (#472)
+ - Fix warnings about `[[likely]]` in `if constexpr`.  (#475)
 7.6.0
  - Removed bad string conversion to `std::basic_string_view<std::byte>`. (#463)
  - Add C++20 concepts: `binary`, `char_string`, `char_strings`.

include/pqxx/field.hxx

@@ -218,8 +218,7 @@ public:
     if (is_null())
     {
       if constexpr (not nullness<T>::has_null)
-        PQXX_UNLIKELY
-      internal::throw_null_conversion(type_name<T>);
+        internal::throw_null_conversion(type_name<T>);
       else return nullness<T>::null();
     }
     else
@@ -494,8 +493,7 @@ template<typename T> inline T from_string(field const &value)
     if constexpr (nullness<T>::has_null)
       return nullness<T>::null();
     else
-      PQXX_UNLIKELY
-    internal::throw_null_conversion(type_name<T>);
+      internal::throw_null_conversion(type_name<T>);
   }
   else
   {

include/pqxx/stream_from.hxx

@@ -338,8 +338,7 @@ inline void stream_from::extract_value(Tuple &t) const
     if constexpr (nullity::has_null)
       std::get<index>(t) = nullity::null();
     else
-      PQXX_UNLIKELY
-    internal::throw_null_conversion(type_name<field_type>);
+      internal::throw_null_conversion(type_name<field_type>);
   }
   else
   {