tests/sys/libc_newlib: drop test

This tests makes little sense to have for a number of reasons:

1. One should not use `iprintf()` for a number of reasons:
    1. It is non-standard and using it over `printf()` makes the code
       less portable (e.g. it cannot be used on AVR)
    2. The idea of adding a leaner variant of `printf()` in addition to
       the larger one is bogus, as apps will end up using both resulting
       in a *larger* firmware instead of a smaller
    3. RIOT's build system already has the `printf_float` module to
       control whether formatting of floating point numbers should be
       suppered. This mechanism will actually result in smaller builds,
       if floating point support is not needed, as it prevents two
       variants of printf to be linked in.
2. The test checks some implementation details (e.g. whether the
   address of two functions is identical), rather than correct behavior
   of the implementation. This is completely bogus.

Author: maribu