[Lettuce deprecation] Remove redundant lettuce tab tests.

These are already covered by bok-choy tests:
  File: test_lms.py
  Class: HighLevelTabTest

lms/djangoapps/courseware/features/high-level-tabs.feature

-@shard_1
-Feature: LMS.All the high level tabs should work
-  In order to preview the courseware
-  As a student
-  I want to navigate through the high level tabs
-
-Scenario: I can navigate to all high - level tabs in a course
-        Given: I am registered for the course "6.002x"
-        And The course "6.002x" has extra tab "Custom Tab"
-        And I am logged in
-        And I click on View Courseware
-        When I click on the tabs then the page title should contain the following titles:
-        | TabName       | PageTitle             |
-        | Courseware    | 6.002x Courseware     |
-        | Course Info   | 6.002x Course Info    |
-        | Custom Tab    | Custom Tab | 6.002x   |
-        | Wiki          | Wiki | edX            |
-        | Progress      | 6.002x Progress       |

lms/djangoapps/courseware/features/high-level-tabs.py

-from lettuce import world, step
-
-
-@step(u'I click on the tabs then the page title should contain the following titles:')
-def i_click_on_the_tab_and_check(step):
-    for tab_title in step.hashes:
-        tab_text = tab_title['TabName']
-        title = tab_title['PageTitle']
-        world.click_link(tab_text)
-        world.wait_for(lambda _driver: title in world.browser.title)
-        assert(title in world.browser.title)