[llvm-readobj][test] - Cleanup testing of the --sections command line option.

We have the `ELF\sections.test` to test --sections.

`ELF\sections.test` uses precompiled objects, it has a bug (does not test -s alias properly).
Also, we test machine specific section types in `ELF\machine-specific-section-types.test`,
so we probably do not need to test `--sections` for a MIPS object in `ELF\sections.test`.
I think it is enough to test ELF32 and ELF64 (we do not test ELF64 in this test).

`Object/readobj-shared-object.test` also tests how llvm-readobj handles
`--sections`. It is location is wrong, it is not complete, it uses precompiled binaries
and it duplicates the `ELF\sections.test` partially (it tests both ELF32 and ELF64).

We have `ELF\readelf-s-alias.test` that tests the `-s` alias for `--sections` in llvm-readobj
and `-s` as an alias for `--symbols` in llvm-readelf.
There is no need to have a separate test for such things.
The test for the `-s` alias for `--sections` can be included into the `ELF\sections.test`.
And the test for `-s` for llvm-readelf is already included into `ELF\symbols.test`.

So, this patch:
1) Removes `Object/readobj-shared-object.test`.
2) Removes `ELF\readelf-s-alias.test`
3) Rewrites the `ELF\sections.test`.
4) Removes ELF/Inputs/trivial.obj.elf-mipsel.

Differential revision: https://reviews.llvm.org/D73686