Remove 'Category' field from policy definition

[jbinko]

Removed the 'Category' field from the policy definition parameters.

[prmerger-automator[bot]]

@jbinko : Thanks for your contribution! The author(s) and reviewer(s) have been notified to review your proposed change.

[learn-build-service-prod[bot]]

Learn Build status updates of commit 4ee875c:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
articles/governance/machine-configuration/how-to/create-policy-definition.md	:white_check_mark:Succeeded

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[v-dirichards]

@michaeltlombardi

Can you review the proposed changes?

Important: When the changes are ready for publication, adding a #sign-off comment is the best way to signal that the PR is ready for the review team to merge.

#label:"aq-pr-triaged" @MicrosoftDocs/public-repo-pr-review-team