tty: remove broken r3964 line discipline

Noone stepped up in the past two years since it was marked as BROKEN by
commit c7084edc3f (tty: mark Siemens R3964 line discipline as BROKEN).
Remove the line discipline for good.

Three remarks:
* we remove also the uapi header (as noone is able to use that interface
  anyway)
* we do *not* remove the N_R3964 constant definition from tty.h, so it
  remains reserved.
* in_interrupt() check is now removed from vt's con_put_char. Noone else
  calls tty_operations::put_char from interrupt context.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210505091928.22010-2-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/char/Kconfig

@@ -218,19 +218,6 @@ config XILINX_HWICAP
 
 	  If unsure, say N.
 
-config R3964
-	tristate "Siemens R3964 line discipline"
-	depends on TTY && BROKEN
-	help
-	  This driver allows synchronous communication with devices using the
-	  Siemens R3964 packet protocol. Unless you are dealing with special
-	  hardware like PLCs, you are unlikely to need this.
-
-	  To compile this driver as a module, choose M here: the
-	  module will be called n_r3964.
-
-	  If unsure, say N.
-
 config APPLICOM
 	tristate "Applicom intelligent fieldbus card support"
 	depends on PCI