client API: remove deprecated qthelper.hpp header

No replacement. Qt or C++ code has no business in this repository, and
new code (even if it uses Qt) should not use it. Get rid of it.

We consider the libmpv API itself as stable. Symbols can be deprecated,
but not be removed. However, qthelper.hpp was never considered part of
the libmpv API. There no ABI implications either, since it's a header-
only implementation that uses C API symbols only. It's just a header
provided for convenience for Qt/C++ programs (i.e. extremely limited
usefulness).

DOCS/client-api-changes.rst

@@ -32,6 +32,16 @@ API changes
 
 ::
 
+ --- mpv 0.33.0 ---
+ 1.107  - Remove the deprecated qthelper.hpp. This was obviously not part of the
+          libmpv API, only an "additionally" provided helper, thus this is not
+          considered an API change. If you are maintaining a project that relies
+          on this header, you can simply download this file and adjust the
+          include statement to use it instead:
+
+            https://raw.githubusercontent.com/mpv-player/mpv/v0.32.0/libmpv/qthelper.hpp
+
+          It is a good idea to write better wrappers for your use, though.
  --- mpv 0.31.0 ---
  1.107  - Deprecate MPV_EVENT_TICK