console: Delete dummy con_font_set() and con_font_default() callback implementations

.con_font_set and .con_font_default callbacks should not pass `struct
console_font *` as a parameter, since `struct console_font` is a UAPI
structure.

We are trying to let them use our new kernel font descriptor, `struct
font_desc` instead. To make that work slightly easier, first delete all of
their no-op implementations used by dummy consoles.

This will make KD_FONT_OP_SET and KD_FONT_OP_SET_DEFAULT ioctl() requests
on dummy consoles start to fail and return `-ENOSYS`, which is intended,
since no user should ever expect such operations to succeed on dummy
consoles.

Suggested-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Peilin Ye <yepeilin.cs@gmail.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/9952c7538d2a32bb1a82af323be482e7afb3dedf.1605169912.git.yepeilin.cs@gmail.com