Remove skip function based on symbol lookup in layout tests

Remove skip function based on symbol lookup in layout tests

[Title] Remove skip function based on symbol lookup in layout tests
[Issue#] N/A
[Problem] Working feature has been skipped in layout tests.
[Cause] Skip function based on symbol lookup causes this happening and it is useless since we have test expectation file.
[Solution] Remove symbol-based skipped list detection.
           - Tools/Scripts/webkitpy/layout_tests/port/webkit.py
             (_missing_symbol_to_skipped_tests)
             (_skipped_tests_for_unsupported_features)
           - Tools/Scripts/webkitpy/layout_tests/port/webkit_unittest.py
             (test_skipped_directories_for_symbols)

Change-Id: I05b3facbbbfb263418dd9ed6bd43e562b09ef3b7