Do not report totalFrames from DAP stackTrace request

Do not report totalFrames from DAP stackTrace request

Currently, gdb will unwind the entire stack in response to the
stackTrace request.  I had erroneously thought that the totalFrames
attribute was required in the response.  However, the spec says:

    If omitted or if `totalFrames` is larger than the available
    frames, a client is expected to request frames until a request
    returns less frames than requested (which indicates the end of the
    stack).

This patch removes this from the response in order to improve
performance when the stack trace is very long.