Remove LoRaWAN license from top-level directory

[geky]

Description

After a discussion with @AnttiKauppila and @hasnainvirk, it looks like the BSD license in the top-level of mbed-os is misplaced. It was originally intended to cover contributions to the LoRaWAN directory, however this is currently covered under the BSD license here.

Having the BSD license at the top-level implies that mbed-os is licensed under BSD, which AFAIK mbed-os is still licensed under Apache 2.0.

Pull request type

[ ] Fix
[x] Refactor
[ ] New target
[ ] Feature
[ ] Breaking change

related #6087
cc @AnttiKauppila, @hasnainvirk, @jlovejoy

[cmonr]

Looks fine to me, but since this is licencing related, will wait until more ppl comment.

[cmonr]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 2630
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7468/

Triggering tests

/morph test
/morph uvisor-test
/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2269
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7468/

[mbed-ci]

Test : SUCCESS

Build number : 2377
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7468/2377