spi-nor: Remove spi_nor::read_id operation

There is currently no useful way to override the default
implementation of this operation.  The returned struct spi_device_id
must have a pointer to struct flash_info in its private data, but this
structure is defined inside spi-nor.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>