Remove unused typedefs

[nlohmann]

This PR removes unused typedefs introduced #2203. Fixes #2221.

[t-b]

@nlohmann Thanks for cleanup after me. ecbb275 (json_pointer::array_index: Use unsigned values for the array index when parsing, 2020-06-20) did not remove those typedefs. It would be nice to have clang-tidy run as part of CI to catch those.

[nlohmann]

@t-b No worries! I am running the latest GCC and Clang to check whether the code emits any warnings. As the compiler flags are not too portable even between GCC versions, I am still hesitant to add this to the CI. I see what I can do.

[coveralls]

Coverage remained the same at 100.0% when pulling fa9f404 on issue2221 into 5ba0f65 on develop.

[t-b]

@nlohmann FYI: At https://github.com/tango-controls/cppTango/blob/tango-9-lts/.travis/gcc-latest/Dockerfile we are using a docker image of the latest (but fixed) GCC version and compile using that. The same approach could be used here. We are also using latest LLVM in docker.