[FIR] Get rid of operation in FirVariableAssignment

`FirVariableAssignment` is creates only for expressions `x = y`
For other assign operators there is a `FirOperatorCall`

compiler/visualizer/render-fir/src/org/jetbrains/kotlin/compiler/visualizer/FirVisualizer.kt

@@ -445,7 +445,7 @@ class FirVisualizer(private val firFile: FirFile) : BaseRenderer() {
         }
 
         override fun visitVariableAssignment(variableAssignment: FirVariableAssignment, data: StringBuilder) {
-            data.append("qualified access: \"${variableAssignment.operation}\"")
+            data.append("variable assignment")
         }
 
         override fun visitStarProjection(starProjection: FirStarProjection, data: StringBuilder) {