Bug 21237: Clean up staff client SCSS

Bug 21237: Clean up staff client SCSS

This patch removes a redundant linter configuration, .sass-lint.yml,
which was added by mistake.

Some linter rules have been modified to better conform to the defacto
standard set by our existing CSS.

staff-global.scss has been corrected to comply with linter rules
(https://github.com/brigade/scss-lint/blob/master/lib/scss_lint/linter/README.md):

- Comment, style: silent
- TrailingSemicolon, true
- Shorthand
- ColorKeyword, enabled: true
- PropertySortOrder

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>