Remove include of backtrace/Backtrace.h.

Somehow the code was still including this include from libbacktrace.
I think the libbacktrace include directory was coming from some
transitive includes. I verified that nothing in debuggerd is using
the libbacktace.so shared library.

Bug: 120606663

Test: Builds, unit tests pass.
Change-Id: I85c2837c5a539ccefc5a7140949988058d21697a