block: Remove blk_find_first/next

block: Remove blk_find_first/next

In [0], Andrew noted a code quality issue in the implementation of
blk_find_first and blk_find_next. This led to the observation that the
logic of these functions was also likely incorrect, and based on a quick
check it seemed the functions were unused outside of test code, which
did not exercise the potential failure case, so we felt they should be
removed. In [1], a test patch which illustrates the failure in sandbox
is provided for reference.

Because a more thorough check agrees that these functions are unused,
they are currently incorrect, and fixed/removable flags on block devices
prior to probe are unreliable, just remove these functions instead of
fixing them. All potential users should have used blk_first_device_err
instead anyway.

CI results at [2].

[0] https://patchwork.ozlabs.org/project/uboot/patch/20250714-blk-uclass-v1-1-d21428c5f762@linaro.org/
[1] https://gist.github.com/gmalysa/b05e73a5c14bc18c5741a0e0e06a2992
[2] https://gitlab.com/gmalysa/lnxdsp-u-boot/-/pipelines/1931210857

Signed-off-by: Greg Malysa <malysagreg@gmail.com>
Reviewed-by: Andrew Goodbody <andrew.goodbody@linaro.org>