n_tty: remove n_tty_receive_char wrapper

n_tty: remove n_tty_receive_char wrapper

The wrapper was meant as an optimization in commits eb3e4668bd9e (n_tty:
Un-inline slow-path n_tty_receive_char()) and e60d27c4d8b3 (n_tty:
Factor LNEXT processing from per-char i/o path). But the current
compiler (gcc 10) inlines it anyway (as expected). Actually, I'm not
sure it ever didn't. It would need to be marked with the noinline
attribute.

So remove this useless wrapper.

And if we ever introduce something similar, we need confirming numbers
first.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210505091928.22010-3-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>