gcp: remove gcp.audit.* fields duplicated in ECS fields

[efd6]

What does this PR do?

This resolves a set of TODOs by removing duplicated fields.

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
• I have verified that Kibana version constraints are current according to guidelines.

Author's Checklist

• [ ]

How to test this PR locally

Related issues

• Closes [gcp]: Remove duplicate fields #3397

Screenshots

[elasticmachine]

Pinging @elastic/security-external-integrations (Team:Security-External Integrations)

[elasticmachine]

💚 Build Succeeded

the below badges are clickable and redirect to their specific view in the CI or DOCS

Expand to view the summary

Build stats

• Start Time: 2022-07-20T22:56:37.308+0000

• Duration: 32 min 30 sec

Test stats 🧪

Test	Results
Failed	0
Passed	46
Skipped	0
Total	46

🤖 GitHub comments

To re-run your PR in the CI, just comment with:

• /test : Re-trigger the build.

[elasticmachine]

🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (4/4)	💚
Files	100.0% (4/4)	💚
Classes	100.0% (4/4)	💚
Methods	93.939% (62/66)	❕
Lines	95.21% (1113/1169)	❕
Conditionals	100.0% (0/0)	💚

[andrewkroh]

@r00tu53r Can you please take a look as this appears related to an earlier change of yours.

[r00tu53r]

@efd6 thank you for addressing the TODOs. LGTM.