serial: Remove CONFIG_SERIAL_MULTI from remaining sources

Remove the parts depending either on disabled CONFIG_SERIAL_MULTI
or ifdefs around CONFIG_SERIAL_MULTI parts since CONFIG_SERIAL_MULTI
is now enabled by default.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Marek Vasut <marek.vasut@gmail.com>
Cc: Tom Rini <trini@ti.com>
Cc: Anatolij Gustschin <agust@denx.de>
Cc: Stefan Roese <sr@denx.de>

post/board/pdm360ng/coproc_com.c

@@ -28,8 +28,6 @@
 #include <post.h>
 #include <serial.h>
 
-#if defined(CONFIG_SERIAL_MULTI)
-
 /*
  * Actually the termination sequence of the coprocessor
  * commands is "\r\n" (CR LF), but here we use a side effect of
@@ -94,4 +92,3 @@ int coprocessor_post_test(int flags)
 
 	return 0;
 }
-#endif	/* CONFIG_SERIAL_MULTI */