[Security Solution] Threshold rules should not add threshold_count to the main signal

[FrankHassanabad]

Describe the bug:

When creating a threshold rule and setting the threshold_count, it should be copying its threshold_count into signal but remove/delete its threshold_count from the main document. During some goodness with refactoring I think this was accidentally dropped in 7.10.0.

When viewing a signal you can see that threshold_count is on the original record as well as within the signal.threshold_count when it should only exist within the signal.

Steps to reproduce:

1. Create a new threshold rule that triggers
2. Look at the signal and notice it has two threshold_count's when it should only have one within signal

Current behavior:
Has a threshold_count at the main document level when it should not.

Expected behavior:
Should not have threshold_count at the main document level.

Kibana/Elasticsearch Stack version:
7.10.0

Server OS version:
any

Browser and Browser OS versions:
any

Elastic Endpoint version:
any

Original install method (e.g. download page, yum, from source, etc.):
any

Functional Area (e.g. Endpoint management, timelines, resolver, etc.):
Detections Alerting