FIR: drop questionable DiagnosticKind.NotRootCause

compiler/fir/tree/src/org/jetbrains/kotlin/fir/diagnostics/ConeSimpleDiagnostic.kt

@@ -52,10 +52,4 @@ enum class DiagnosticKind {
     WrongLongSuffix,
 
     Other,
-
-    /**
-     * Special diagnostic kind that is caused by issues at other places in the code. Such diagnostics should not be reported so that user
-     * sees less noise.
-     */
-    NotRootCause
 }