cli: Remove RESULTCLASS summary field

The RESULTCLASS summary field allowed the benchmark to define how a
resultclass should be summarised; either arithmetic mean or harmonic
mean. harmonic mean was preferred for rates. But this was slow to
calculate, and not all integrated test suites (e.g. pts) provides the
information. So let's rip it out and simplify the code a bit, and use
arithmetic mean always.

This improves speed of "result show"; for a resultstore containing ~9000
results, total time (most of which is Pandas load time) decreased from
3.1 seconds to 2.8 seconds.

Since we have a lot of CSV data that contains this attribute, let's
include a hack to automatically remove it, allowing us to continue to
load the data. We are not compatible with existing containers though so
they will need to be rebuilt.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>