flash: spi_nor: remove `SPI_NOR_IDLE_IN_DPD`

Remove `SPI_NOR_IDLE_IN_DPD` to simplify the possible transition states
in the `spi_nor` driver. This option was originally added 5 years ago
when device runtime PM was in a much less mature state.

I do not believe having a separate power management implementation for
this one in-tree driver is in the interests of the project.

The behaviour of `SPI_NOR_IDLE_IN_DPD` leads to extremly suboptimal
behaviour in use cases where multiple small reads are performed
sequentially, see #69588.

Removal of this option does not break the behaviour of any existing
applications, only increases current consumption in idle by ~10uA until
device runtime PM is enabled.

Signed-off-by: Jordan Yates <jordan@embeint.com>