Vers SQL: Refactoring in system fields init

This is preliminary for Syntax for creating TRX_ID-based tables #510.

Removed redundant proxying:

• Table_scope_and_contents_source_st::vers_check_system_fields() +
  Vers_parse_info::check_sys_fields()
  are now just Table_scope_and_contents_source_st::vers_check_system_fields();
• vers_create_sys_field() + vers_create_sys_field()
  are now just Vers_parse_info::create_sys_field().

Moved vers_create_sys_field() to Vers_parse_info::create_sys_field().