lib: utils/serial: remove semihosting_putc

For some debuggers that do not implement SYSWRITEC and SYSREADC
operations, we have to use SYSWRITE and SYSREAD.

Instead of implementing semihosting_putc() using SYSWRITE, let us
simply remove semihosting_putc() because console_putc/console_puts
are now interchangeable.

Signed-off-by: Chen Pei <cp0613@linux.alibaba.com>
Signed-off-by: Xiang W <wxjstz@126.com>
Reviewed-by: Guo Ren <guoren@kernel.org>
Reviewed-by: Anup Patel <anup@brainfault.org>