Remove dead code and simplify redundant constructs

[guillaume-sainthillier]

• remove the unused Doctrine FieldMapping imports left over from the
  DBAL 3/4 compatibility work
• remove AdminContextFactory's duplicated getUser() call and the unused
  $request parameter of getDashboardDto()
• remove the unused $entityDto parameter of
  CommonPreConfigurator::buildTemplatePathOption()
• return expressions directly instead of assigning them to a variable
  used only once (SearchDto, MenuFactory, MenuItemMatcher,
  ComparisonType)
• flatten the nested null/Stringable checks in
  CrudDto::getDefaultPageTitle() and use the nullsafe operator in
  AdminContextFactory