Exporting 5.12 content fails because of duplicate 'shared' folder in examples

QtMultimedia examples missing, were present in Qt5.11.1. This is known issue for first 5.12 snapshot.

After https://codereview.qt-project.org/#/c/232532/ creating combined examples package is failing because there is now 'shared' folder under examples in 2 different submodules (qtdoc and qtmultimedia) and export tool failed because of that. To solve this issue properly we need either change the folder structure of combined examples package or remove conflicting 'shared' folders