bsc#1184131: use /etc/login.defs for shadow configuration

[imobachgs]

Back in 2019, we added support to read/write the shadow configuration from /usr/etc and /etc (see bsc#1155735) for further details. This change was wrongly included in SLE 15 SP2 (and Leap 15.2): the shadow package included in that version only reads the settings from /etc/login.defs.

This PR partially reverts those changes, so YaST will only take /etc/login.defs into account.

• Replace CFA::ShadowConfig with CFA::LoginDefs in the ShadowConfig moduile (used by yast2-users).
• Drop CFA::ShadowConfig. It is only used in yast2-security. See https://github.com/yast/yast-security/pull/108.

Fixes bsc#1184131.

[coveralls]

Coverage decreased (-0.4%) to 30.739% when pulling 25c9d7eb844776de9aef0939a6f1cc62a1f7d765 on bsc-1184131 into e914214656d217ae05a51f6347e22b520caae907 on SLE-15-SP2.