Revert "mem: spi-mem: add declaration for spi_mem_default_supports_op"

Revert "mem: spi-mem: add declaration for spi_mem_default_supports_op"

We have a duplicate declaration of spi_mem_default_supports_op() which
was added twice, first with commit af6266c1c27a ("mem: spi-mem: add
declaration for spi_mem_default_supports_op") for v2021.04, and again
with commit 2299076e34f8 ("spi: spi-mem: export
spi_mem_default_supports_op()") for v2021.07.

The first commit is reverted here, because the second better matches the
definition and has a better place in the declaration order.

Note: Linux declares this in a different section of spi-mem.h which is
disabled in U-Boot through `#ifndef __UBOOT__`.

This reverts commit af6266c1c27add8beac7f3365c00b3525a9012c4.

Signed-off-by: Alexander Dahl <ada@thorsis.com>