Remove /v1/features/{feature_id}/stats/wpt/browsers/{browser}/channels/{channel}/{metric_view} endpoint

[jcscottiii]

Background:

• The endpoint /v1/features/{feature_id}/stats/wpt/browsers/{browser}/channels/{channel}/{metric_view} was created to simplify the database query when looking at a single feature. It removes the unneeded SUM aggregation as well that is used in /v1/stats/wpt/browsers/{browser}/channels/{channel}/{metric_view}. Now with Make subtest_count the default for timeseries data #189, the query building has been reduced to a shared template between the two endpoints and it knows when to add the SUM aggregation function or not. Now it seems the separation at the API layer was a premature optimization and it is no longer needed.

Desired work:

• Remove /v1/features/{feature_id}/stats/wpt/browsers/{browser}/channels/{channel}/{metric_view} from openapi and any functions associated with it
• Add a special case to the database call to check if there is a singular feature, then render the template without the SUM aggregate function by toggling IsSingleFeature to true.
• Have the frontend call /v1/stats/wpt/browsers/{browser}/channels/{channel}/{metric_view} instead and pass in a single feature_key

This will be useful to have before we share the openapi document.