Simplify attribute traits

[saki7]

Part of #1
cc: @yaito3014

The old implementation had unnecessary class Context parameter for many core traits, which were never being used since its very first introduction 12 years ago. We will not support such premature customization points in X4 and they were considered the contribution barrier for improving core components.

The only remaining attributes in X4 that are deduced dynamically at parse time are the classes that are controlled by traits::pseudo_attribute.

• traits::attribute_of: removed in favor of parser_traits<Parser>::attribute_type
• traits::has_attribute: removed in favor of parser_traits<Parser>::has_attribute
• traits::attribute_type: removed; fusion ADT adapters will not be supported in X4
• traits::pseudo_attribute: moved to core/attribute.hpp
• static bool has_attribute = ...;: redundant definition removed from parsers, only the truly needed ones remain
• static bool handles_container = ...;: redundant definition removed from parsers, only the truly needed ones remain
• static bool is_pass_through_unary = true;: removed in favor of struct x4::proxy_parser