Hide ability to create alert on custom metrics dashboard

Summary

Custom metrics dashboard currently does not support alerts (#194267 (closed)). Attempting to create an alert renders a weird frontend bug of undefined. A page refresh does make it go away but we should just hide the alert option in the dropdown so that a user does not stumble upon this unsupported feature.

Steps to reproduce

1. Create a custom metrics dashboard
2. Click on the dropdown associated to a chart on the custom metrics dashboard
3. Click on Alerts
4. Fill in the alerts fields and click add
5. Notice the odd undefined message

Example Project

https://ops.gitlab.net/gitlab-com/metrics-dogfooding/-/environments/266/metrics?dashboard=.gitlab%2Fdashboards%2Fuptime.yml&duration_seconds=604800

What is the current bug behavior?

(What actually happens)

What is the expected correct behavior?

(What you should see instead)

Relevant logs and/or screenshots

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Output of checks

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Results of GitLab environment info

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sudo gitlab-rake gitlab:env:info)
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sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production)

Results of GitLab application Check

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(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:check SANITIZE=true)
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sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true)
(we will only investigate if the tests are passing)

Possible fixes

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