common: Remove unused CONFIG_FIT_SHAxxx selectors

Originally CONFIG_FIT_SHAxxx enabled specific SHA algos for and only
for has_calculate() in common/image-fit.c. However, since commit
14f061dc ("image: Drop IMAGE_ENABLE_SHAxxx"),
the correct selector was changed to CONFIG_SHAxxx.

The extra "_FIT_" variants are neither used, nor needed. Remove them.
One defconfig disables FIT_SHA256, which is now changed to 'SHA256'.

CMD_MVEBU_BUBT needs to select select SHA256 to avoid undefined
references to "sha256_*()". bubt.c needs sha256, so this selection is
correct. It is not clear why this problem did not manifest before.

Note that SHA selection in SPL is broken for this exact reason. There
is no corresponding SPL_SHAxxx. Fixing this is is beyond the scope of
this change.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>