[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/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 {