remove 'bl_iot_sdk/customer_app/get-start' folder

[ntn888]

remove the 'bl_iot_sdk/customer_app/get-start'.

because of it's name its the first folder I look at. BUT:

• it uses the outdated API;
• makefile scripts has no support for the new bl70x chip

and it confuses the new user...

I wasted a lot of time because of this to compile my first program for the bl702!