spi_nor: remove unneeded enter_dpd() at end of spi_nor_configure()

This block of code was in the original commit that added
CONFIG_SPI_NOR_IDLE_IN_DPD but later modifications
added acquire_device() and release_device() calls
earlier in spi_nor_configure() and the release_device()
function will have already done the enter_dpd().

Signed-off-by: Mike J. Chen <mjchen@google.com>