[VFX] Hide eye dropper from the ColorField

This is a small one liner fix to remove the problematic eye dropper, reported in [UUM-64983](https://jira.unity3d.com/browse/UUM-64983). Fixing this would be too costly resource-wise for such a small issue, so we decided simply to hide the button. 

**Before:**
<img width="700" alt="before" src="https://media.github.cds.internal.unity3d.com/user/1762/files/a718b893-e41a-4444-8304-858a61b66b45">

**After:**
<img width="704" alt="after" src="https://media.github.cds.internal.unity3d.com/user/1762/files/d9782151-02b3-4bff-ab0a-49c7abd1c212">

Author: JMargevics

Packages/com.unity.visualeffectgraph/Editor/UIResources/uxml/VFXFilterWindow.uxml

@@ -40,6 +40,7 @@
             <ui:VisualElement name="ColorFieldRow">
                 <uie:ColorField name="CategoryColorField"
                                 label="Set Custom color"
+                                show-eye-dropper="false"
                                 value="#FFFFFF"
                                 />
                 <ui:Button name="ResetButton"