vt: selection, remove redeclaration of poke_blanked_console

It is declared in vt_kern.h, so no need to declare it in selection.c
which includes the header.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20200219073951.16151-8-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>