e_tracesig used to be implemented for Darwin compat. Nowadays the Darwin
compatiblity layer is gone and there are no other users.
This functionality isn't used where it shall be used in the existing
codebase.
If we want to emulate debugging interfaces in compat layers we would need
to implement that from scratch anyway. We would need to be bug compatible
with other OSes too.
Proposed on tech-kern@.
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