[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/testData/symbols/symbolByPsi/explicitBackingField.kt

@@ -0,0 +1,6 @@
+var p: Int
+    field = "test"
+    get() = field.length
+    set(value) {
+        field = value.toString()
+    }