[FIR] Remove the redundant backing field check

This commit fixes a failing visibility check
(FirSymbolByPsiTestGenerated.testBackingField) for the backing field.

Merge-request: KT-MR-6389
Merged-by: Nikolay Lunyak <Nikolay.Lunyak@jetbrains.com>

analysis/analysis-api-impl-base/tests/org/jetbrains/kotlin/analysis/api/impl/base/test/cases/symbols/AbstractSymbolByPsiTest.kt

@@ -6,6 +6,7 @@
 package org.jetbrains.kotlin.analysis.api.impl.base.test.cases.symbols
 
 import org.jetbrains.kotlin.analysis.api.KtAnalysisSession
+import org.jetbrains.kotlin.psi.KtBackingField
 import org.jetbrains.kotlin.psi.KtDeclaration
 import org.jetbrains.kotlin.psi.KtFile
 import org.jetbrains.kotlin.psi.KtParameter
@@ -28,6 +29,7 @@ abstract class AbstractSymbolByPsiTest : AbstractSymbolTest() {
     private val KtDeclaration.isValidForSymbolCreation
         get() =
             when (this) {
+                is KtBackingField -> false
                 is KtParameter -> !this.isFunctionTypeParameter
                 else -> true
             }