Fix 'new[]' array freed with 'delete'

[szsam]

Summary of changes

The array _scratch_buf is allocated using new[] in line 761 of mbed-os/storage/kvstore/securestore/source/SecureStore.cpp. But it was freed using delete.
Fix the bug by freeing _scratch_buf with delete[].

Impact of changes

Migration actions required

Documentation

None

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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

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[mbed-ci]

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