drivers: spi_nor: Remove unused SPI_NOR_{PAGE,SECTOR}_SIZE symbols

Unused since commit 2a590d3f ("drivers/spi_nor: remove configurability
of page/sector/block sizes"). The help texts already say they're unused,
but it probably doesn't hurt to remove them as well.

Found with a script.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>