[BUG] Remove reference to `mc config host add`

[ravindk89]

minio/mc#5206

A user noted that this command is referenced in the mc admin quickstart, and is also missing from the deprecation list.

We should both:

• Remove all references to the command (which was deprecated pre-2020)
• Add a simple reference to the 'deprecations' listing with the note that it was removed in the most current mc release

[feorlen]

fyi @toastsandwich