cli: Refactor to create join_results() and filter_results()

Introduce new utility functions `join_results()` and `filter_results()`
in fastpath/utils/table.py to encapsulate this shared logic. The former
takes the dictionary of resultstore tables and returns a single results
table with much of the common metadata joined in. The latter filters the
results table by sut/swprofile/benchmark/resultclass.

Update all previous call sites to use this new utility, reducing
redundancy and improving maintainability.

As a beneficial side effect, we clean up display of the resultclass in
the dashboard so remove the units. Units are still displayed on the
Y-axis of the graph.

Co-developed-by: Aishwarya TCV <aishwarya.tcv@arm.com>
Signed-off-by: Aishwarya TCV <aishwarya.tcv@arm.com>
Co-developed-by: Ryan Roberts <ryan.roberts@arm.com>
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>