[Obs AI Assistant] Remove `get_apm_timeseries` and update `summarize` function description

[mdbirnstiehl]

• the get_apm_timeseries tool needs to be removed since it is deprecated (I had added it to the ticket later so maybe it was missed)

• the summarize tool is incorrectly described. This is the description we have in the codebase:

description: `Use this function to store facts in the knowledge database if the user requests it.
An embedding will be created that you can recall later with a semantic search.
When you create this summarisation, make sure you craft it in a way that can be recalled with a semantic
search later, and that it would have answered the user's original request., descriptionForUser:' 
This function allows the Elastic Assistant to summarize things from the conversation.',

also, for summarize we should mention that this tool is only available if the knowledge base is already installed