This patch does the following:

1. Makes Field_vers_trx_id::type_handler() return
  &type_handler_vers_trx_id rather than &type_handler_longlong.
  Fixes Item_func::convert_const_compared_to_int_field() to
  test field_item->type_handler() against &type_handler_vers_trx_id,
  instead of testing field_item->vers_trx_id().

2. Removes VERS_TRX_ID related code from
  Type_handler_hybrid_field_type::aggregate_for_comparison(),
  because "BIGINT UNSIGNED GENERATED ALWAYS AS ROW {START|END}"
  columns behave just like a BIGINT in a regular comparison,
  i.e. when not inside AS OF.

3. Removes
   - Type_handler_hybrid_field_type::m_vers_trx_id;
   - Type_handler_hybrid_field_type::m_flags;
  because a "BIGINT UNSIGNED GENERATED ALWAYS AS ROW {START|END}"
  behaves like a regular BIGINT column when in UNION.

4. Removes Field::vers_trx_id(), Item::vers_trx_id(), Item::field_flags()
  They are not needed anymore. See N1.