mmc_xpt: Remove dubious end of mmc_print_ident

The end of this function finishes the passed in sbuf, calls printf
manually on the contents, and then clears it. The caller then tries
to print the resulting sbuf. This works currently but will not work
for future callers that pass in an external sbuf to be appended to.

Reviewed by: mav
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D41260