This reverts commit 6388a6e3.

With migration GlFormTextarea away from BFormTextarea, we can also
remove the BSV form-textarea.js file, as well as a couple unused mixins:

- form-selection
- form-validity
- form-text

This reverts commit 1d862516.

Remove BFormTextarea from GlFormTextarea

See merge request !5807



Merged-by: Thomas Hutterer <thutterer@gitlab.com>
Approved-by: Thomas Hutterer <thutterer@gitlab.com>
Reviewed-by: Mark Florian <mflorian@gitlab.com>
Reviewed-by: Peter Hegman <phegman@gitlab.com>
Reviewed-by: Thomas Hutterer <thutterer@gitlab.com>
Reviewed-by: GitLab Duo <gitlab-duo@gitlab.com>
Co-authored-by: Lorenz van Herwaarden <lvanherwaarden@gitlab.com>