DevTools: Remove redundant default destructor and forward declarations

Problem:
- Default destructors (and constructors) are in `.cpp` files. This
  prevents the compiler's optimizer from inlining them when it thinks
  inlining is appropriate (unless LTO is used).
- Forward declarations can prevent some optimizations, such as
  inlining of constructors and destructors.

Solution:
- Remove them or set them to `= default` and let the compiler handle
  the generation of them.
- Remove unneeded forward declarations.

Userland/DevTools/Inspector/InspectorServerClient.h

@@ -0,0 +1,38 @@
+/*
+ * Copyright (c) 2018-2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <InspectorServer/InspectorClientEndpoint.h>
+#include <InspectorServer/InspectorServerEndpoint.h>
+#include <LibIPC/ServerConnection.h>
+
+namespace Inspector {
+
+class InspectorServerClient final
+    : public IPC::ServerConnection<InspectorClientEndpoint, InspectorServerEndpoint>
+    , public InspectorClientEndpoint {
+    C_OBJECT(InspectorServerClient);
+
+public:
+    virtual void handshake() override
+    {
+        greet();
+    }
+
+    virtual ~InspectorServerClient() override = default;
+
+private:
+    InspectorServerClient()
+        : IPC::ServerConnection<InspectorClientEndpoint, InspectorServerEndpoint>(*this, "/tmp/portal/inspector")
+    {
+        handshake();
+    }
+
+    virtual void dummy() override { }
+};
+
+}