ShaderGraph: remove duplicate LIGHTMAP_ON and DIRLIGHTMAP_COMBINED variants for builtin-deferred

Unnecessary duplicate variants may be created due to the simultaneous usage of 
- `multi_compile_prepassfinal`
- `LIGHTMAP_ON` and `DIRLIGHTMAP_COMBINED` in different lines.

for deferred mode in built-in RP.

This PR removes the usage of `LIGHTMAP_ON` and `DIRLIGHTMAP_COMBINED` to prevent the creation of these duplicate variants.

[POI-828 [Shader System] Shader Variant Management](https://jira.unity3d.com/browse/POI-828)

Author: sindharta-tanuwijaya

Packages/com.unity.shadergraph/Editor/Generation/Targets/BuiltIn/Editor/ShaderGraph/Targets/BuiltInLitSubTarget.cs

@@ -572,8 +572,6 @@ static class LitKeywords
 
             public static readonly KeywordCollection Deferred = new KeywordCollection
             {
-                { CoreKeywordDescriptors.Lightmap },
-                { CoreKeywordDescriptors.DirectionalLightmapCombined },
                 { CoreKeywordDescriptors.MainLightShadows },
                 { CoreKeywordDescriptors.ShadowsSoft },
                 { CoreKeywordDescriptors.LightmapShadowMixing },