Remove `--oss` flag from ECS generator script

[ebeahan]

Summary

Remove the --oss flag as an option in the ECS generator script, the oss-fallback module, and remove the --oss section in USAGE.md.

Background

The --oss flag was added (#991) to enable users to fallback to OSS-compatible Elasticsearch types when possible.

The feature was deprecated (#1232) when (as of 7.11) an Apache 2.0 licensed distribution of Elasticsearch was no longer available with intent to remove in the next ECS major version.