Clean-up: TDBStore no longer requires BlockDevice to have flash behaviour

[LDong-Arm]

Summary of changes

Since the merging of #11987, TDBStore works with any BlockDevice without requiring flash behaviour (i.e. erasing bytes to the erase value, FF). This PR

• cleans up unnecessary wrapping of FlashSimBlockDevice throughout Mbed OS
• updates any outdated design docs and Doxygen around flash simulation

Impact of changes

None.

Migration actions required

None.

Documentation

ARMmbed/mbed-os-5-docs#1386
(Not interdependent on this PR, but for the same goal)

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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

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

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Reviewers

@ARMmbed/mbed-os-core

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[ciarmcom]

@LDong-Arm, thank you for your changes.
@ARMmbed/mbed-os-core @ARMmbed/mbed-os-maintainers please review.

[mergify]

This PR cannot be merged due to conflicts. Please rebase to resolve them.

[mergify]

This PR cannot be merged due to conflicts. Please rebase to resolve them.

[evedon]

Looks good.

[LDong-Arm]

Rebased

Pull request has been modified.

[0xc0170]

CI started

[mbed-ci]

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jenkins-ci/mbed-os-ci_build-example-GCC_ARM	✔️
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