Fix selected for User#commit_email input

What does this MR do?

This fixes the input for User#commit_email so the correct option is selected. Previously if no database value was set, a default value (User#email) was set as the selected option. We don't want to set a database value without user input, so this removes the default value.

This is a follow-up to !68591 (merged). That removed the first mechanism for setting an input value without any user action. This removes the second (and final) mechanism.

Note: This is similar to the recent change to User#notification_email, but more refactoring has been required for User#commit_email.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Related to https://gitlab.com/groups/gitlab-org/-/epics/6564