[xla:gpu] NFC: Delete unused external allocations and allocate/free command buffer commands
[xla:gpu] NFC: Delete unused external allocations and allocate/free command buffer commands

Run time memory allocation inside command buffers does not compose well with XLA static memory planning and was never fully implemented. Instead we should build it around memory spaces and introduce a special memory space for virtual allocations (via direct use of virtual memory allocation APIs or CUDA graphs).
