Remove unnecessary imports from Timer.idl (#12895)

* Remove unnecessary imports from Timer.idl

With Timer.idl now being imported in ReactCoreInjection.idl, keeping the imports of Timer.idl limited to non-Fabric things helps keep builds that aren't using Fabric simpler.

* Change files

Author: rozele

change/react-native-windows-eb09148a-fb13-460d-8861-37532b00f584.json

@@ -0,0 +1,7 @@
+{
+  "type": "prerelease",
+  "comment": "Remove unnecessary imports from Timer.idl",
+  "packageName": "react-native-windows",
+  "email": "[email protected]",
+  "dependentChangeType": "patch"
+}

vnext/Microsoft.ReactNative/Timer.idl

@@ -2,9 +2,7 @@
 // Licensed under the MIT License.
 
 #include "DocString.h"
-import "IJSValueReader.idl";
-import "IJSValueWriter.idl";
-import "Theme.idl";
+import "IReactPropertyBag.idl";
 
 namespace Microsoft.ReactNative {