`mbed dm`: Remove references to internal script from help
`mbed dm`: Remove references to internal script from help

### Description

Now that we are delegating the help text to `device_managment.py`,
it should print out help as if it's invoked with `mbed device-management`.
This PR changes the argument parser and the help text to do just that.

### Pull request type

    [x] Fix
    [ ] Refactor
    [ ] Target update
    [ ] Functionality change
    [ ] Breaking change

kjbracey-arm/mbed,mbedmicro/mbed,betzw/mbed-os,kjbracey-arm/mbed,betzw/mbed-os,mbedmicro/mbed,c1728p9/mbed-os,andcor02/mbed-os,kjbracey-arm/mbed,betzw/mbed-os,betzw/mbed-os,betzw/mbed-os,betzw/mbed-os,c1728p9/mbed-os,c1728p9/mbed-os,andcor02/mbed-os,andcor02/mbed-os,andcor02/mbed-os,andcor02/mbed-os,c1728p9/mbed-os,mbedmicro/mbed,c1728p9/mbed-os,mbedmicro/mbed,c1728p9/mbed-os,kjbracey-arm/mbed,andcor02/mbed-os,mbedmicro/mbed