K2: drop obsolete TODO about multiple WasExperimental

In certain situations like constructor calls, it's possible
to have different WasExperimental/SinceKotlin pairs associated
with one use-site, e.g. a constructor call is associated both
with a constructor declaration and a class declaration.
However, looks like it make no sense to handle this situation
in FirSinceKotlinHelpers, because FirOptInUsageBaseChecker already
handles them.

#KT-59825 Fixed

compiler/tests-common-new/tests-gen/org/jetbrains/kotlin/test/runners/DiagnosticTestGenerated.java

@@ -43112,6 +43112,12 @@ public class DiagnosticTestGenerated extends AbstractDiagnosticTest {
                 runTest("compiler/testData/diagnostics/testsWithStdLib/experimental/wasExperimental.kt");
             }
 
+            @Test
+            @TestMetadata("wasExperimentalCombined.kt")
+            public void testWasExperimentalCombined() throws Exception {
+                runTest("compiler/testData/diagnostics/testsWithStdLib/experimental/wasExperimentalCombined.kt");
+            }
+
             @Test
             @TestMetadata("wrongTargetsWithoutExplicitTarget.kt")
             public void testWrongTargetsWithoutExplicitTarget() throws Exception {