Remove long-deprecated "target count" and "target number" commands.

Given that the manual states that these two subcommands are
deprecated and were scheduled to be removed back in 2010,
remove them and the corresponding documentation from the
manual.

Change-Id: Iaac633349d7fcb8b7f964109c7d26dd0cc5fc233
Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Reviewed-on: http://openocd.zylin.com/1860
Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>