accel/amdxdna: Remove amdxdna_flush()

accel/amdxdna: Remove amdxdna_flush()

amdxdna_flush() was introduced to ensure that the device does not access
a process address space after it has been freed. However, this is no
longer necessary because the driver now increments the mm reference count
when a command is submitted and decrements it only after the command has
completed. This guarantees that the process address space remains valid
for the entire duration of command execution. Remove amdxdna_flush to
simplify the teardown path.

Reviewed-by: Mario Limonciello (AMD) <superm1@kernel.org>
Signed-off-by: Lizhi Hou <lizhi.hou@amd.com>
Link: https://patch.msgid.link/20251216031311.2033399-1-lizhi.hou@amd.com