Remove supertarget

[theotherjimmy]

Description

This was used for Doxygen in the past, but now the Doxygen uses a
whitelist of directories to scan. I think it makes sense to remove this
if only to avoid confusion.

Pull request type

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

[adbridge]

@theotherjimmy Edited the PR header to how it should be. Please refrain from changing the structure in the future as this will break the auto release labeling script going forward. :)

[0xc0170]

/morph build

[theotherjimmy]

@adbridge I did not change anything, i used hub

[adbridge]

@theotherjimmy Then you may have to stop using 'hub' if it breaks our PR header flow

[mbed-ci]

Build : SUCCESS

Build number : 1546
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/6440/

Triggering tests

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

[mbed-ci]

Exporter Build : SUCCESS

Build number : 1185
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6440/

[mbed-ci]

Test : SUCCESS

Build number : 1325
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/6440/1325