Remove support for sending e-mails via SMTP

[php-coder]

Extracted from #935 (comment):

• application*.properties: remove spring.mail.* properties
• application*.properties: exclude MailSenderAutoConfiguration
• ansible: remove user_mail_password variable
• pom.xml: consider removing dependency on spring-boot-starter-mail

Tech debt for: #935

[0pdd]

@php-coder the puzzle #1092 is still not solved.