vulkan/command: nuke command queue

With the switch to timeline semaphores, and the resulting absence of the unsignaling mechanism, we no longer have any reason to hold on for commands longer than necessary. As such, we can immediately submit all commands to the GPU, rather than delaying such submission until the user flushes queued commands explicitly.

This completely gets rid of the command queue, the command object associations (which were only needed for partial flushing), the last_cmd state tracking and the relevant API functions.

No performance difference (as expected), but vastly simpler code.