Cleanup GTM API: make functions static, remove dead code

Cleanup GTM API: make functions static, remove dead code

The cleanup does two basic things:

* Functions used only in a single source file are made static (and also
  removed from the header file, of course). This reduces the size of the
  public GTM API.

* Unused functions (identified by the compiler thanks to making other
  functions static in the previous step) are removed. The assumption is
  that this code was not really tested at all, and would only make
  future improvements harder.