target: stm8: drop include file stm8.h

target: stm8: drop include file stm8.h

The file stm8.h is only included by stm8.c and provides some basic
declaration that can be simply part of the C file.

Drop the file stm8.h and move its content in stm8.c
Replace the macro 'STM8_NUM_CORE_REGS' with the existing macro
'STM8_NUM_REGS'.

While there:
- drop the useless include of "hello.h".

Change-Id: Iecd1a27f0630cdbbfd51033d34aa3d468aa63464
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8856
Tested-by: jenkins
Reviewed-by: zapb <dev@zapb.de>