As the discussion on our mailing list showed, we do not get any
performance gain from using unordered_map over a QHash.

Removing the ifdef code simplifies the code and also prevents us
from accidentally committing C++11 features into the current branches.

We should also not loose too much by not compiling with C++11 anymore,
considering that Qt 4.x does not use any C++11 features yet, besides
Q_DECL_CONSTEXPR for QFlags. Furthermore we do not use STL in (m)any
places either, where we could get potential performance benefits.
Thus C++11 flag will be enabled later, after reaching a consensus on
our mailing list.