snap: Force usage of libsecret everywhere and remove password manager service plug

[3v1n0]

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[foundry376-bot]

This pull request has been mentioned on Mailspring Community. There might be relevant details there:

https://community.getmailspring.com/t/password-management-error-after-the-latest-upgrade/7298/33

[3v1n0]

Oh this may require https://github.com/snapcore/snapcraft/pull/4477/ wondering if using it without "=" but a space instead works anyways.