spi: remove struct spi_message::is_dma_mapped

spi: remove struct spi_message::is_dma_mapped

There are no more users of the deprecated is_dma_mapped in struct
spi_message so it can be removed.

References in documentation and comments are also removed.

A few similar checks if xfer->tx_dma or xfer->rx_dma are not NULL are
also removed since these are now guaranteed to be NULL because they
were previously set only if is_dma_mapped was true.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://msgid.link/r/20240325-spi-remove-is_dma_mapped-v2-1-d08d62b61f1c@baylibre.com
Signed-off-by: Mark Brown <broonie@kernel.org>