Don't reset encryption choice when disabling encryption controls.

Without this, the mail.identity.default.encryptionpolicy pref does not
work: adding a key to an account always set its encryptionpolicy to 0
no matter the desired default.

While we're at it, also kill two lines of code before a call to
enableEncryptionControl() that exactly replicates it.

https://bugzilla.mozilla.org/show_bug.cgi?id=1674995