remoteproc: qcom: pas: Drop unused dtb_mem_region field

dtb_mem_region is no longer referenced after the ioremap was moved
to respective places where mapping is required. Remove it from
struct qcom_pas.

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Reviewed-by: Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>
Signed-off-by: Mukesh Ojha <mukesh.ojha@oss.qualcomm.com>
Link: https://lore.kernel.org/r/20260724182858.1868271-6-mukesh.ojha@oss.qualcomm.com
Signed-off-by: Bjorn Andersson <andersson@kernel.org>