Delete old redirect files in docs

doc/development/prometheus.md

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-redirect_to: '../user/project/integrations/prometheus.md'
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-This document was moved to [another location](../user/project/integrations/prometheus.md).
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-<!-- This redirect file can be deleted after February 1, 2021. -->
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doc/user/packages/workflows/monorepo.md

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-redirect_to: '../npm_registry/index.md'
-disqus_identifier: 'https://docs.gitlab.com/ee/user/packages/workflows/monorepo.html'
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-This document was moved to [another location](../npm_registry/index.md).
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-<!-- This redirect file can be deleted after <2021-02-14>. -->
-<!-- Before deletion, see: https://docs.gitlab.com/ee/development/documentation/#move-or-rename-a-page -->

doc/user/project/milestones/burndown_charts.md

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-redirect_to: 'burndown_and_burnup_charts.md'
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-This document was moved to [another location](burndown_and_burnup_charts.md).
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-<!-- This redirect file can be deleted after February 1, 2021. -->
-<!-- Before deletion, see: https://docs.gitlab.com/ee/development/documentation/#move-or-rename-a-page -->