automated: linux: ltp: skipfile-lkft: Remove the all_boards alias

Previously the `all` keyword for boards was replaced with an list
`all_boards` to store the list of supported boards to make it possible
to remove boards as part of the automation for skipfile testing.

To make the commits easier to read, it has been suggested that the
all_boards alias should be removed, and the list of boards should be
maintained within each skipfile entry. This way when boards are removed
as part of the automated skipfile testing the diff will be much easier
to read. When a board is removed from an entry that previously had all
boards, instead of the diff reading "all_boards" -> "<list of all_boards
with some entries removed>" the diff will just show which boards were
removed.

This will make the update commits much easier to interpret and review.
Additionally, as the automated skipfile testing is run and boards are
removed based on the test results, the all_boards variable will likely
be removed in most places in the skipfile, as most tests will likely
work on at least one board. For these reasons, it seems sensible to
remove the alias and variable.