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'.

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>