Remove all code deprecated for removal in 4.0.0

Now that 4.x is the branch for backward-compatibility breaks, we should remove all the code that was deprecated for removal in domain:4.0.0, across every submodule.

domain

• domain.module: domain_uri() (use Domain::uri()) and template_preprocess_domain_nav_block() (use DomainThemeHooks::preprocessDomainNavBlock()).
• DomainNegotiator and DomainNegotiatorInterface: the deprecated request/hostname/scheme accessors and the DOMAIN_MATCHED_* constants (use DomainResolver and DomainNegotiationContext).
• DomainStorage and DomainStorageInterface: normalizeHostname(), getRequestHostname(), getRequestScheme() and the machine-name helper (use DomainResolver and MachineName::fromString()).
• DomainInterface and Domain: matchPathPrefix() (use DomainResolver::matchPathPrefix()).
• DomainValidator and DomainValidatorInterface: use entity constraint validation instead.

domain_access

• domain_access.module: domain_access_confirm_fields() (use DomainAccessHelperInterface::confirmFields()).
• DomainAccessManager: getAccessValues(), getAllValue(), getDefaultValue(), clearStaticCache() and userCanAccessDomain() (use the DomainAccessFields utility).

domain_source

• domain_source.module: domain_source_confirm_fields(), domain_source_get_source_domain() and domain_source_get() (use DomainSourceHelperInterface).

domain_alias

• DomainAliasValidator and DomainAliasValidatorInterface: use entity constraint validation instead.

domain_config

• DomainConfigOverrideEditable: the deprecated override-diff helper.

domain_config_ui

• DomainConfigUIManager::getActiveDomainId() and the matching interface method (use DomainNegotiationContext::getDomainId() or DomainConfigUIManagerInterface::getActiveEditingDomainId()).

Also in this issue

• Sweep the test suite to use the container object ($this->container->get()) instead of \Drupal::service().
• Remove any backward-compatibility shims and legacy aliases that only exist to support the deprecated API listed above.

Related: #3612201 (the 4.x entity-storage refactor) and #3612441 (the phpunit fixes).