luci-app-acl: remove support for plaintext login passwords

Upon reviewing the rpcd session support code, it turns out that account
definitions with plaintext password values are not supported, the password
value is treated as crypt() hash/salt in any case.

Ref: https://github.com/openwrt/luci/issues/5179
Signed-off-by: Jo-Philipp Wich <jo@mein.io>