Change the text color of a message used in time tracking component

What does this MR do?

• Resolves the following comment from !45490 (merged)

@uhlexsis commented

There was only one small thing I noticed. The No estimate or time spent empty state looks like it is maybe #303030 when it should be #666666 . Pretty small change. :)

• Removes two components because they only contained templates and it's good to avoid creating vue instances.
  • time_tracking/estimate_only_pane.vue
  • time_tracking/no_tracking_pane.vue

The removed components have been inlined in time_tracker.vue

Screenshots (strongly suggested)

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Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #273555 (closed)