[llvm-readobj] - Remove Error.cpp,.h and drop dependencies in the code.

We have Error.cpp/.h which contains some code for working with error codes.
In fact we use Error/Expected<> almost everywhere already and we can get rid
of these files.

Note: a few places in the code used readobj specific error codes,
e.g. `return readobj_error::unknown_symbol`. But these codes are never really used,
i.e. the code checks the fact of a success/error call only.
So I've changes them to `return inconvertibleErrorCode()` for now.
It seems that these places probably should be converted to use `Error`/`Expected<>`.

Differential revision: https://reviews.llvm.org/D86772