BufferQueue: remove freeAllBuffersExceptHeadLocked()

Now that we are having separate buffer-instances for the buffer-
queue, we can free all buffers; we don't have to keep the head
alive.

Change-Id: I023e9161a2501d99333f8868ce438afa914ec50f
Signed-off-by: Lajos Molnar <lajos@google.com>
Related-to-bug: 7093648