chore(cleanup): remove import token feature flag

[yhabib]

Motivation

The nns-dapp uses feature flags to introduce new features into the main branch without disrupting existing flows. Some of these feature flags have been in the code for a long time, so we want to remove them.

This PR removes the feature flags introduced in #6209.

Changes

• Removes ENABLE_IMPORT_TOKEN_BY_URL feature flag.
• Eliminates tests that check the UI behavior when the feature flag is off.
• Updates tests to reflect the UI behavior when the feature flag is on, as this is the default behavior.

Tests

• The pipeline should pass as before.
• Tested in devenv

Todos

• Add entry to changelog (if necessary).
  Not necessary.

[mstrasinskis]

LGTM, thank you!