usb: usbtmc: Remove sysfs group TermChar and auto_abort

As all the properties of the usbtmc driver can now be
controlled on a per file descriptor basis by ioctl functions
the sysfs interface is of limited use.
We are not aware about applications that are using the sysfs
parameter TermChar, TermCharEnabled or auto_abort.

Signed-off-by: Guido Kiener <guido.kiener@rohde-schwarz.com>
Reviewed-by: Steve Bayless <steve_bayless@keysight.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>