IPCCompiler: Remove invalid state from IPC message classes

At some point, we stopped ever constructing invalid messages. This makes
that clearer, and will allow us to stop requiring that IPC arguments be
default-constructible.

Libraries/LibIPC/Message.h

@@ -69,7 +69,6 @@ class Message {
     virtual u32 endpoint_magic() const = 0;
     virtual int message_id() const = 0;
     virtual char const* message_name() const = 0;
-    virtual bool valid() const = 0;
     virtual ErrorOr<MessageBuffer> encode() const = 0;
 
 protected:

Meta/Lagom/Tools/CodeGenerators/IPCCompiler/main.cpp

@@ -345,7 +345,6 @@ public:)~~~");
    typedef class @message.response_type@ ResponseType;)~~~");
 
     message_generator.appendln(R"~~~(
-    @message.pascal_name@(decltype(nullptr)) : m_ipc_message_valid(false) { }
     @message.pascal_name@(@message.pascal_name@ const&) = default;
     @message.pascal_name@(@message.pascal_name@&&) = default;
     @message.pascal_name@& operator=(@message.pascal_name@ const&) = default;
@@ -412,12 +411,8 @@ public:)~~~");
     })~~~");
 
     message_generator.appendln(R"~~~(
-    virtual bool valid() const override { return m_ipc_message_valid; }
-
     virtual ErrorOr<IPC::MessageBuffer> encode() const override
     {
-        VERIFY(valid());
-
         IPC::MessageBuffer buffer;
         IPC::Encoder stream(buffer);
         TRY(stream.encode(endpoint_magic()));
@@ -445,8 +440,7 @@ public:)~~~");
     }
 
     message_generator.appendln(R"~~~(
-private:
-    bool m_ipc_message_valid { true };)~~~");
+private:)~~~");
 
     for (auto const& parameter : parameters) {
         auto parameter_generator = message_generator.fork();
@@ -715,8 +709,6 @@ class @endpoint.name@Stub : public IPC::Stub {
                     message_generator.appendln(R"~~~(
             [[maybe_unused]] auto& request = static_cast<const Messages::@endpoint.name@::@message.pascal_name@&>(message);
             auto response = @handler_name@(@arguments@);
-            if (!response.valid())
-                return Error::from_string_literal("Failed to handle @endpoint.name@::@message.pascal_name@ message");
             return make<IPC::MessageBuffer>(TRY(response.encode()));)~~~");
                 }
             } else {