Drops prebuild bootloaders not supported in Mbed OS 6

[VeijoPesonen]

Target support for these targets has been removed with PRs #12810 and #12775.

Summary of changes

Target support for these board has been removed so there is no need to keep these prebuild bootloader binaries.

Impact of changes

Migration actions required

Documentation

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Pull request type

[X] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[X] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers

@SeppoTakalo
@MarceloSalazar
@0xc0170

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[0xc0170]

CI started

[mbed-ci]

Test run: FAILED

Summary: 2 of 3 test jobs failed
Build number : 1
Build artifacts

Failed test jobs:

• jenkins-ci/mbed-os-ci_build-ARM
• jenkins-ci/mbed-os-ci_build-GCC_ARM

[0xc0170]

Ci restarted (not reported properly)

[mbed-ci]

Test run: SUCCESS

Summary: 6 of 6 test jobs passed
Build number : 2
Build artifacts