Remove unnecessary _summaries filter in getActivitiesByTraceId

The filter is not needed because summary entries are stored under a
different key prefix ([...keyPrefix, "_summaries", traceId]) which
won't match when listing with prefix [...keyPrefix, traceId].

https://github.com/fedify-dev/fedify/pull/502#discussion_r2643066630
https://github.com/fedify-dev/fedify/pull/502#discussion_r2643077208