Remove SMITH_USE_DFEM option

[chapman39]

from an older conversation:

it seems like SMITH_USE_DFEM isn't associated with any package, but rather its to enable or disable certain tests/ examples. maybe SMITH_ENABLE_DFEM is more appropriate?

i still don't fully understand the difference between dfem and enzyme, so maybe this question doesn't make any sense, but could we use SMITH_USE_ENZYME here?

also last question, could this option be removed once #1477 merges?

Originally posted by @chapman39 in #1465 (comment)