Lora: Change visibility of LoRaMac internal methods as private

[kivaisan]

Description

Some LoRaMac methods were defined as public even though there were used only
internally.

Also removed definition of some methods which were not even implemented nor used.

Pull request type

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

[kivaisan]

@AnttiKauppila @hasnainvirk please review.

[0xc0170]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 2855
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7839/

Triggering tests

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

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2485
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7839/

[mbed-ci]

Test : SUCCESS

Build number : 2614
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7839/2614

[cmonr]

@kjbracey-arm Mind taking a quick look at this so that we can wrap up the PR? It's even ready to be merged assuming it's all good!