drm/qxl: remove unused variable from `qxl_process_single_command()`

drm/qxl: remove unused variable from `qxl_process_single_command()`

Clang 14 in an (essentially) defconfig loongarch64 build for next-20240327
reports [1]:

     drivers/gpu/drm/qxl/qxl_ioctl.c:148:14: error: variable 'num_relocs'
     set but not used [-Werror,-Wunused-but-set-variable]

The variable was originally used in the `out_free_bos` label, but commit
74d9a6335dce ("drm/qxl: Simplify cleaning qxl processing command")
removed the use that happened in that label.

Thus remove the unused variable.

Fixes: 74d9a6335dce ("drm/qxl: Simplify cleaning qxl processing command")
Closes: https://lore.kernel.org/lkml/CANiq72kqqQfUxLkHJYqeBAhpc6YcX7bfR96gmmbF=j8hEOykqw@mail.gmail.com/ [1]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://lore.kernel.org/r/20240327175556.233126-2-ojeda@kernel.org
Signed-off-by: Maxime Ripard <mripard@kernel.org>