[SRPF] Hide RenderMode field and remove documentation

RenderMode was never supported by URP. Maybe it was planned to be added but I couldn't find anyone who knows anything about.
Instead we hide the field and remove documentation for it.

Author: kirill-titov-u

Packages/com.unity.render-pipelines.universal/Documentation~/light-component.md

@@ -54,10 +54,6 @@ The Light Inspector includes the following groups of properties:
 
 | Property:| Function: |
 |:---|:---|
-| **Render Mode**| Use this drop-down to set the rendering priority of the selected Light. This can affect lighting fidelity and performance (refer to the **Performance Considerations**, below). |
-|    **Auto**| The rendering method is determined at run time, depending on the brightness of nearby lights and the current [Quality](https://docs.unity3d.com/Manual/class-QualitySettings.html) settings. |
-|    **Important**| The light is always rendered at per-pixel quality. Use **Important** mode only for the most noticeable visual effects (for example, the headlights of a player’s car). |
-|    **Not Important**| The light is always rendered in a faster, vertex/object light mode.  |
 | **Culling Mask**| Use this to selectively exclude groups of objects from being affected by the Light. For more information, refer to [Layers](https://docs.unity3d.com/Manual/Layers.html).|
 
 ## <a name="Shadows"></a>Shadows

Packages/com.unity.render-pipelines.universal/Editor/Lighting/UniversalRenderPipelineLightUI.Drawers.cs

@@ -267,8 +267,6 @@ static void DrawEmissionContent(UniversalRenderPipelineSerializedLight serialize
 
         static void DrawRenderingContent(UniversalRenderPipelineSerializedLight serializedLight, Editor owner)
         {
-            serializedLight.settings.DrawRenderMode();
-
             if (serializedLight.settings.light.type != LightType.Rectangle &&
                 !serializedLight.settings.isCompletelyBaked)
             {