[FIR] Get rid of operation in FirVariableAssignment

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

compiler/fir/tree/src/org/jetbrains/kotlin/fir/FirRenderer.kt

@@ -928,7 +928,7 @@ class FirRenderer(builder: StringBuilder) : FirVisitorVoid() {
         visitQualifiedAccess(variableAssignment)
         variableAssignment.lValue.accept(this)
         print(" ")
-        visitAssignment(variableAssignment.operation, variableAssignment.rValue)
+        visitAssignment(FirOperation.ASSIGN, variableAssignment.rValue)
     }
 
     override fun visitArraySetCall(arraySetCall: FirArraySetCall) {

compiler/fir/tree/src/org/jetbrains/kotlin/fir/expressions/FirVariableAssignment.kt

@@ -25,7 +25,6 @@ abstract class FirVariableAssignment : FirPureAbstractElement(), FirQualifiedAcc
     abstract override val calleeReference: FirReference
     abstract val lValue: FirReference
     abstract val rValue: FirExpression
-    abstract val operation: FirOperation
 
     override fun <R, D> accept(visitor: FirVisitor<R, D>, data: D): R = visitor.visitVariableAssignment(this, data)
 

compiler/fir/tree/src/org/jetbrains/kotlin/fir/expressions/impl/FirVariableAssignmentImpl.kt

@@ -8,7 +8,6 @@ package org.jetbrains.kotlin.fir.expressions.impl
 import org.jetbrains.kotlin.fir.FirSourceElement
 import org.jetbrains.kotlin.fir.expressions.FirAnnotationCall
 import org.jetbrains.kotlin.fir.expressions.FirExpression
-import org.jetbrains.kotlin.fir.expressions.FirOperation
 import org.jetbrains.kotlin.fir.expressions.FirVariableAssignment
 import org.jetbrains.kotlin.fir.impl.FirAbstractAnnotatedElement
 import org.jetbrains.kotlin.fir.references.FirReference
@@ -22,8 +21,7 @@ import org.jetbrains.kotlin.fir.visitors.*
 class FirVariableAssignmentImpl(
     override val source: FirSourceElement?,
     override var safe: Boolean,
-    override var rValue: FirExpression,
-    override var operation: FirOperation
+    override var rValue: FirExpression
 ) : FirVariableAssignment(), FirModifiableQualifiedAccess, FirAbstractAnnotatedElement {
     override val annotations: MutableList<FirAnnotationCall> = mutableListOf()
     override var explicitReceiver: FirExpression? = null

compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/ImplementationConfigurator.kt

@@ -383,7 +383,6 @@ object ImplementationConfigurator : AbstractFirTreeImplementationConfigurator()
 
         impl(variableAssignment) {
             parents += modifiableQualifiedAccess
-            isMutable("operation")
             lateinit("calleeReference")
             defaultNoReceivers()
 

compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/NodeConfigurator.kt

@@ -439,7 +439,6 @@ object NodeConfigurator : AbstractFieldConfigurator() {
         variableAssignment.configure {
             +field("lValue", reference)
             +field("rValue", expression).withTransform()
-            +field("operation", operationType)
         }
 
         whenSubjectExpression.configure {