Remove assertion in KDecorationUnstable's constructor.
Follow-up to c146941, which stopped exporting KDecorationBridgeUnstable.
The way the typeinfo and object comparisons are performed in a dynamic_cast
are not entirely defined in any ABI spec (or the C++ standard), and the
situation gets trickier when a hidden symbol (KDecorationBridgeUnstable)
gets passed between a program such as kwin or kcmshell4 and a dlopen()ed
library like kwin3_laptop.so: kwin is unaware of the symbol in
kwin3_laptop.so and vice-versa, so stricter implementations of the C++
runtime can choose to make the dynamic_cast call fail and return 0.
This was a source of crashes on GCC for a long time  until its
developers chose to make the typeinfo comparison in libstdc++ >= 4.5.x less
strict and just perform a string comparison of the type names by default.
It does not make things more correct, though, and after c146941 KWin started
crashing when built in debug mode and run with libcxxrt, another C++ runtime
implementation, since it performed a stricted typeinfo comparison by
Simply removing the dynamic_cast should be fine: it is in an assertion, so
the code is not essential, and the assertion itself is not entirely correct.
Additionally, the assertion has already been removed from the master branch
in commit 4702bbd.