Stop checking for the append-script marker in filesystem operations

[microbit-carlos]

Right now, it is constantly checking the micropython hex string to see if it has the old marker to append a script to a known flash address: :::::::::::::::::::::::::::::::::::::::::::

It's not a lot of processing, but the hex strings can be large and we are trying to increase performance.

The hex string provided to the MicropythonFsHex constructor should be correctly formed, and that should be the responsibility of the caller (like the Python Editor).

[microbit-carlos]

Done in a418ade.