spi: Remove unnecessary #ifdefs in header file

These prevent use of compile-time checks such as:

    if (CONFIG_IS_ENABLED(DM_SPI))

since, for example, if CONFIG_SPL_DM_SPI is not enabled then the
definitions are not included by spi.h and the C code will not build.

The #ifdefs are unnecessary since there are no conflicts with the pre-DM
code. In any case we have almost switched over to driver model for SPI.

Drop these #ifdefs from spi.h to fix a build warning on chromebook_coral
in the following patch.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>