Remove extra ARMc6 instance in unique mapping

[cmonr]

Description

This recent commit (https://github.com/ARMmbed/mbed-os/pull/7302/files#diff-8ae3dde46b55f3a922724cd3585922b2) added ARMc6 which ends up multiplicly defining ARMc6 in what is suppose to be a set with unique entries.

Pull request type

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

[cmonr]

/morph build

[cmonr]

/morph build

[mbed-ci]

Build : FAILURE

Build number : 2638
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7544/

[mbed-ci]

Build : ABORTED

Build number : 2639
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7544/

[cmonr]

Going to try this again since CI is now quiet.

/morph build

[cmonr]

@ARMmbed/mbed-os-maintainers Thoughts about bringing this in without CI?
@ARMmbed/mbed-os-test

Details: The original CI problem was that when the mbed compile -S or other -S commands are used, the CI build pipeline fails before the PR can even be applied. Because of this, no PRs will be aleo to run until this PR is merged in, which creates a cyclical dependency.
I've opened a ticket in Jira asking for a possible Jenkins change/update, but wanted to discuss this alternative as well.

[cmonr]

Different note: A failed export job appears to indicate that IAR builds are having network and/or machine issues: http://mbed-os.s3-eu-west-1.amazonaws.com/builds/exporter/7479/FAIL/K82F/make_iar/86491627bf80d1b5e5d3b92dabc242d556304e0c_exporter_build_log_K82F_make_iar.txt

[mbed-ci]

Build : FAILURE

Build number : 2641
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7544/

[0xc0170]

CI IAR issue still needs to be fixed anyway, I'll talk to @OPpuolitaival.

I would like to have this green asap

[0xc0170]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 2642
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7544/

Triggering tests

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

[mbed-ci]

Test : SUCCESS

Build number : 2381
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7544/2381

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2273
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7544/