[FIR] 2/2 Cleanup: drop no longer necessary FirActualCallableDeclarationChecker

It became possible to drop it after KT-62590. Now, on a frontend, the
return type check is part of a common AbstractExpectActualChecker logic

Change in nestedAnnotationClassViaActualTypealias.fir.kt aligns the
behaviour with K1. KT-61964

Review: https://jetbrains.team/p/kt/reviews/12750/timeline

compiler/testData/multiplatform/incompatibleCallables/output.txt

@@ -6,7 +6,7 @@ Output:
 Exit code: COMPILATION_ERROR
 Output:
 compiler/testData/multiplatform/incompatibleCallables/jvm.kt:1:12: error: 'actual fun f1(): String' has no corresponding expected declaration
-The following declaration is incompatible:
+The following declaration is incompatible because return type is different:
     expect fun f1(): Unit
 
 actual fun f1(): String = ""