Remove building of doc snippets on Android

The snippets are built to ensure they remain in a compilable state.
Unfortunately Android's install target installs them to /lib which is not
desirable. The remaining targets are sufficient to check the state.

Task-number: QTBUG-37638
Change-Id: I4bd1b58ad6d7e5bfbb8c1e08d48834049ec9d48e
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>