adb: kill adb_mutex_t, adb_cond_t.

Now that we have support for std::mutex and std::condition_variable on
Windows, remove our mutex compatibility layer in favor of the C++ one.

Bug: http://b/31653591
Test: mma && $ANDROID_HOST_OUT/nativetest64/adb_test/adb_test && \
      python test_adb.py && python test_device.py
      (also on Windows)

Change-Id: I5b7ed9c45cc2a32edcf4e77b56dc28e441f15f34