[MSE] move "removeSourceBuffer()" from sourcebuffers to mediasource

Since  activesourcebuffers contains reference of parts of sourcebuffers, I think when I call sourcebuffers.remove() to remove a source buffer, the buffers referenced in activesourcebuffers are also deleted at the same time.

So what's the problem here not moving the remove() to mediasource object?

Yang
Huawei