Checking gpgsm.info Revision 4847, there is no explanation how
to delete a secret key for gpgsm.
It is true that this is rarely used operation, but there are real use cases.
Just today I had a user that somehow made a mistake while importing
a secret key and now did not have the password anymore.
Even when he had the .p12 to import the secret key again, he could not
change or set a new password. So during the import attempt I saw
the file name in ~/.gnupg/private-keys-v1.d/ and manually deleted the file.
Now a reimport worked and the new password was set.
I suggest to put a short explanation in the documentation
why there is no explicit --delete-secret-keys command
and how to do it manually in the rare cases this must be done.