[Profiling, 9.0.0] Remove unused fields from the stacktraces and flamegraph API

[rockdaboot]

Several fields in the API responses are unused meanwhile but need to be kept in order to allow upgrading 8.x clusters.

UNUSED fields in stacktraces response
total_frames indicates the number of frames including the inline frames. UNUSED in Kibana.
stack_trace_events is a map of stack trace IDs to the number of events of the stack trace. UNUSED in Kibana since 8.12 (replaced by stack_traces.*.count).

UNUSED fields in flamegraph response
TotalCPU is UNUSED in Kibana.
CountInclusive[0] is the same as SelfCPU (index 0 is for the root frame).
(Both values are a sum of CountExclusive[1..].)

[UPDATE]
A previous description wrongly said
TotalSamples is the same as SelfCPU.
This is not true!
TotalSamples is the number of events (sum of the event.count values), while SelfCPU only contains the number of events that we have stacktraces for.

[elasticsearchmachine]

Pinging @elastic/obs-knowledge-team (Team:obs-knowledge)