[llvm] [SandboxIR][NFC] Delete SandboxIR.h (PR #110309)

[Farzon Lotfi via llvm-commits]

farzonl wrote:

I'm seeing build failures because of this change.

```
D:\project\llvm-project\llvm\include\llvm/SandboxIR/Instruction.h(1100,9): error: base class 'SingleLLVMInstructionImpl<llvm::UnaryInstruction>' has private constructor
 1100 |       : SingleLLVMInstructionImpl(ID, Opc, LLVMI, Ctx) {}
      |         ^
D:\project\llvm-project\llvm\include\llvm/SandboxIR/Instruction.h(362,3): note: implicitly declared private here
  362 |   SingleLLVMInstructionImpl(ClassID ID, Opcode Opc, llvm::Instruction *I,
      |   ^
```

https://github.com/llvm/llvm-project/pull/110309