remoteproc: imx_rproc: Remove the assignement to method

remoteproc: imx_rproc: Remove the assignement to method

'method' is no longer used in imx_rproc.c, so remove the assignment.
But imx_dsp_rproc.c is still using 'method', so still keep the field
in struct imx_rrpoc_dcfg.

No functional changes.

Reviewed-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Daniel Baluta <daniel.baluta@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Link: https://lore.kernel.org/r/20251024-imx_rproc_c4-v4-4-af83ed3fdbba@nxp.com
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>