[FIR] Fix more tests because of modifier usage in resolution

compiler/testData/diagnostics/tests/operatorsOverloading/kt11300.fir.kt

@@ -8,8 +8,6 @@ class A {
 fun main() {
     val a = A()
     <!OPERATOR_MODIFIER_REQUIRED!>a[1]<!>++
-    //fir prefers plusAssign call if neither get+set nor plusAssign resolverd
-    //hence UNRESOLVED_REFERENCE here
     a[1] <!UNRESOLVED_REFERENCE!>+=<!> 3
     <!OPERATOR_MODIFIER_REQUIRED!>a[1]<!> = a[1] + 3
 }