Removal of Symbian support

Qt Creator's support for Symbian was at its peak in version
2.4.x. Nobody really verified it in Qt Creator 2.5 or 2.6.
It is most likely rotten. Let's remove it!

Also, the Symbian support code was spread throughout the whole
Qt Creator code base. The plugin interfaces evolved in the
meantime and target platforms like Android or QNX have 99% of
their code in separate plugins.

In case anyone wants to revive Symbian support in Qt Creator,
please create a plugin for it.

Change-Id: I56a758a3e2fd5b8c64d9aeb8f63d8e916c4883be
Reviewed-by: Alessandro Portale <alessandro.portale@nokia.com>