x86/callthunks: Delete unused "struct thunk_desc"

It looks like it was never used.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org
Link: https://lore.kernel.org/r/843bf596-db67-4b33-a865-2bae4a4418e5@p183