Remove unneeded pipeline barrier in vulkan

We don't need to syncronize the mapped memory writes to the buffer since
all CPU writes are already syncronized when we submit a command buffer. And we are using coherent memory for buffers so we don't need to call vkFlushMappedMemory

BUG=skia:

Review-Url: https://codereview.chromium.org/2289973002
1 file changed