Fix mc_project_version_delete() return value

The function's return type does not match the WSDL definition.

This is a regression introduced by a80f447687722b52160d00c96c21a5be5f4329e1
which changed version_delete()'s return type from true (bool) to void
Since mc_project_version_delete() actually returned that, it causes a
side effect because void == NULL == false.

To avoid an unnecessary and potentially breaking change in the WSDL, the
function now returns true.

Fixes 0025856

(cherry picked from commit 5039c7b86a21f41a64c904e3eb16e0eb25ac2cb0)