using an idea from dtardon:

<dtardon> noelgrandin, hi. could you try to run the unusedmethods clang
plugin with "make build-nocheck"? that would catch functions that are
only used in tests. e.g., i just removed the whole o3tl::range class,
which has not been used in many years, but htere was a test for it...
<noelgrandin> dtardon, interesting idea! Sure, I can do that.

Change-Id: I5653953a426a2186a1e43017212d87ffce520387
Reviewed-on: https://gerrit.libreoffice.org/22041Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>