cmake: Use official FindBacktrace rather than custom FindExecInfo

cmake: Use official FindBacktrace rather than custom FindExecInfo

Newer versions of CMake ship their own FindBacktrace.cmake module,
which replaces the FindExecInfo.cmake module so far shipped by
Quassel. The official module is also a bit more thorough in terms
of cross-platform support.

To avoid duplication, remove Quassel's custom find module and use
the official one instead.