os-helpers-sb: Silence command when checking for user_mode_enabled

[mtoman]

is_secured will use command to check whether user_mode_enabled is defined, which was added to prevent the error message user_mode_enabled: command not found as this is only defined on EFI systems. However now on EFI systems it echoes user_mode_enabled each time the command is found which is also not what we want.

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[alexgg]

This has been merged in https://github.com/balena-os/meta-balena/commit/0cc0e51ec48fd90c7164cf458c6a2b583319999d