[RPW] Switch to BuiltIn only when we plan to do it

https://jira.unity3d.com/browse/UUM-67156

Before this change when we switched from URP to HDRP or vice versa we set globalRenderPipeline for the Shader systems twice.

First we set `Shader.globalRenderPipeline = string.Empty` from destructor of the previous RP so we go from RP to BuiltIn. 
Then when we create RP we set `Shader.globalRenderPipeline = "RPName"` and we go from BuiltIn to RP.
Each set of the globalRenderPipeline tag call `void Shader::SetGlobalRenderPipeline(const core::string& name) {}` and reload shaders.

Now if we switch from any SRP to any SRP we will avoid intermediate step where we set an empty string to the `Shader.globalRenderPipeline` by checking `GraphicsSettings.currentRenderPipeline`

Author: kirill-titov-u

Packages/com.unity.render-pipelines.high-definition/Runtime/RenderPipeline/HDRenderPipeline.cs

@@ -742,9 +742,6 @@ public void ResetRTHandleReferenceSize(int width, int height)
 
         void SetRenderingFeatures()
         {
-            // Set sub-shader pipeline tag
-            Shader.globalRenderPipeline = k_ShaderTagName;
-
             // HD use specific GraphicsSettings
             m_PreviousLightsUseLinearIntensity = GraphicsSettings.lightsUseLinearIntensity;
             GraphicsSettings.lightsUseLinearIntensity = true;
@@ -860,8 +857,6 @@ bool IsSupportedPlatformAndDevice(out GraphicsDeviceType unsupportedGraphicDevic
 
         void UnsetRenderingFeatures()
         {
-            Shader.globalRenderPipeline = string.Empty;
-
             GraphicsSettings.lightsUseLinearIntensity = m_PreviousLightsUseLinearIntensity;
             GraphicsSettings.lightsUseColorTemperature = m_PreviousLightsUseColorTemperature;
             GraphicsSettings.useScriptableRenderPipelineBatching = m_PreviousSRPBatcher;

Packages/com.unity.render-pipelines.universal/Runtime/UniversalRenderPipeline.cs

@@ -248,8 +248,6 @@ public UniversalRenderPipeline(UniversalRenderPipelineAsset asset)
             XRSystem.SetDisplayMSAASamples(msaaSamples);
             XRSystem.SetRenderScale(asset.renderScale);
 
-            Shader.globalRenderPipeline = k_ShaderTagName;
-
             Lightmapping.SetDelegate(lightsDelegate);
 
             CameraCaptureBridge.enabled = true;
@@ -316,8 +314,6 @@ protected override void Dispose(bool disposing)
 
             pipelineAsset.DestroyRenderers();
 
-            Shader.globalRenderPipeline = string.Empty;
-
             SupportedRenderingFeatures.active = new SupportedRenderingFeatures();
             ShaderData.instance.Dispose();
             XRSystem.Dispose();