[llvm-readobj][test] - Remove amdgpu-elf-definitions.test

`amdgpu-elf-definitions.test` is a file that tests
`Format`, 'OS/ABI' and 'Machine' fields printed for EM_AMDGPU.
It uses a precompiled object.

It is not needed to have it, because:
1) We already test formats in `file-header-format.test` + `llvm/unittests/Object/ELFObjectFileTest.cpp`.
2) We test 'OS/ABI' in `file-header-os-abi.test` for all possible values.
3) We test machine types in `file-header-machine-types.test`

In addition we have `amdgpu-elf-headers.test` which tests EM_AMDGPU specific flags. I.e.
it also tests the fact that llvm-readobj/elf are able to dump such objects.

This patch removes the test case and the corresponding binary.

Differential revision: https://reviews.llvm.org/D86774