[FIR2IR] Drop Fir2IrSymbols in favor of IrPublicSymbol...Impl

Effectively, those two symbol hierarchies have the same semantics, so
  there is no sense to keep them both

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/symbols/impl/IrPublicSymbolBase.kt

@@ -107,3 +107,6 @@ class IrFieldPublicSymbolImpl(sig: IdSignature, descriptor: PropertyDescriptor?
 class IrTypeParameterPublicSymbolImpl(sig: IdSignature, descriptor: TypeParameterDescriptor? = null) :
     IrBindablePublicSymbolBase<TypeParameterDescriptor, IrTypeParameter>(sig, descriptor),
     IrTypeParameterSymbol
+
+class IrScriptPublicSymbolImpl(sig: IdSignature, descriptor: ScriptDescriptor? = null) :
+    IrBindablePublicSymbolBase<ScriptDescriptor, IrScript>(sig, descriptor), IrScriptSymbol