ui: Drop colors defined in Bootstrap 5.3

- They were only present for compatibility with Bootstrap 5.2. The values were
picked directly from Bootstrap 5.3. So, there is no change in output.

Tests:

- Go to user creation form and make errors. A form element with error has red
outline like more. The help text and error message are also in red like before.
In dark mode, they are more appropriate with the rest of the page and with
higher contrast.

- On default styled button, background color on hover is also improved
similarly.

- The border for select-all button in the create new backup page is also
improved similarly.

Reviewed-by: James Valleroy <jvalleroy@mailbox.org>

static/themes/default/css/main.css

@@ -8,11 +8,6 @@
     --progress-color: #3498db; /* Blue */
     --freedombox-navbar-color: white;
 
-    /* Missing variables in Bootstrap 5.2 copied from Bootstrap 5.3 */
-    --bs-secondary-bg: #e9ecef;
-    --bs-form-invalid-color: #dc3545;
-    --bs-form-invalid-border-color: #dc3545;
-    --bs-secondary-border-subtle: #c4c8cb;
 }
 
 /*