Fixed partly qualified UITK controls types in com.unity.shadergraph IdentifierField

[axel-dubillet]

Purpose of this PR

Remove partly qualified UI Toolkit controls types in IdentifierField. These types were moved from UnityEditor.UIElements to UnityEngine.UIElements and the ScriptUpdater is having a hard time dealing with partly qualified moved types.

Another solution was to fully qualify the types, but this would have broken the package for older versions, where the types are still in UnityEditor.UIElements

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Testing status

Opening projects using com.unity.shadergraph, which would hang while importing without the fix.

Previously ran full ABV pipeline against trunk and errors look like instabilities (timeouts and errors unrelated to my changes which are isolated in com.unity.shadergraph).

The errors reported in Shader Graph tests are false errors and are also happening on the master branch here.

[github-actions]

Hi! This comment will help you figure out which jobs to run before merging your PR. The suggestions are dynamic based on what files you have changed.
Link to Yamato: https://yamato.cds.internal.unity3d.com/jobs/902-Graphics
Search for your PR branch using the sidebar on the left, then add the following segment(s) to the end of the URL (you may need multiple tabs depending on how many packages you change)

Shader Graph
/.yamato%252Fall-shadergraph.yml%2523PR_ShaderGraph_2021.2
Depending on your PR, you may also want
/.yamato%252Fall-shadergraph_builtin_foundation.yml%2523PR_ShaderGraph_BuiltIn_Foundation_2021.2
/.yamato%252Fall-shadergraph_builtin_lighting.yml%2523PR_ShaderGraph_BuiltIn_Lighting_2021.2

Depending on the scope of your PR, you may need to run more jobs than what has been suggested. Please speak to your lead or a Graphics SDET (#devs-graphics-automation) if you are unsure.

[theopnv]

Removing myself from reviewers as I have no knowledge of the actual code. I'm still subscribed to the PR to keep an eye on it.

[jessebarker]

LGTM, but I would like to see CI run on a trunk pipeline before this gets merged.