Revert "Fill in cn_name in struct consdev."

This reverts commit 82a21151cf1d7a3e9e95b9edbbf74ac10f386d6a.

This commit was made to aid debugging before consoles are initialised so
that they can more easily be identified from a debugger. However,
various consoles (cfecons, ofwcons, mambocons and rcons) all check
whether cn_name[0] is non-zero to see they are attached or not, and so
this breaks that (perhaps misguided) approach. On RISC-V this results in
rcons (the SBI firmware console driver) racing with the real UART driver
and so input gets probabilistically lost on the real UART (around 2/3 of
the time for me on QEMU).

Moreover, the name given to CONSOLE_DRIVER isn't necessarily the same as
what eventually gets written to cn_name, such as cfecons vs cfe, rcons
vs riscv, ttyj0 vs aju, ttyv0 vs sc, ttyuN vs uart_phyp/opal and all
manner of things vs ucom, so in some cases this is in fact misleading as
the name will change after attaching.

Discussed with: cperciva