This PR removes own protection as OpenPGP.js config.minRSABits implicitly has the default value of 2047.

close #4905
close #5631

Tests (delete all except exactly one):

• Unit tests are already in place:
  [KeyUtil.diagnose] decrypts and tests secure PK and insecure SK
[unit][OpenPGPKey.parse] sets usableForEncryption and usableForSigning to false for RSA key less than 2048
[unit][OpenPGPKey.parse] sets usableForEncryption to false and usableForSigning to true for 2048/RSA PK and 1024/RSA SK
[unit][OpenPGPKey.decrypt] sets usableForEncryption to false and usableForSigning to true for 2048/RSA PK and 1024/RSA SK

To be filled by reviewers

I have reviewed that this PR... (tick whichever items you personally focused on during this review):

• addresses the issue it closes (if any)
• code is readable and understandable
• is accompanied with tests, or tests are not needed
• is free of vulnerabilities
• is documented clearly and usefully, or doesn't need documentation