Remove the conversion to bool from json_view

explicit operator bool meant "refers to a value", which silently differs
from what a basic_json converts to. Use !v.is_discarded() instead.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/api/basic_json_view/index.md

@@ -62,7 +62,6 @@ element access, iteration, `get<T>()`, JSON Pointer support, `dump()`, or compar
 - [**is_primitive**](is_primitive.md) - return whether the type is primitive
 - [**is_structured**](is_structured.md) - return whether the type is structured
 - [**is_discarded**](is_discarded.md) - return whether the view is invalid
-- [**operator bool**](operator_bool.md) - return whether the view refers to a value
 
 ### Capacity