minor #1607 [Bootstrap v5] Remove the custom form style (rosier)

This PR was merged into the main branch.

Discussion
----------

[Bootstrap v5] Remove the custom form style

Most of the custom style has no effect since the form elements don't have a `.form-group` class anymore.

And the layout of textarea fields is broken by:
```css
.form-control {
  height: 45px;
}
```

Since bootstrap already has style for the input fields that is almost identical I opted for removing the custom style

Commits
-------

19ec620 [Bootstrap v5] Remove the custom form style

Author: javiereguiluz

assets/styles/admin.css

@@ -1,7 +1,3 @@
-:root {
-  --blue: #007bff;
-}
-
 /* Page: 'Backend post index'
    ------------------------------------------------------------------------- */
 body#admin_post_index .item-actions {
@@ -24,11 +20,3 @@ body#admin_post_show .post-tags .label-default {
 body#admin_post_show .post-tags .label-default i {
   color: #95A6A7;
 }
-
-.form-control {
-  border-width: .125rem;
-}
-.form-control:focus {
-  color: var(--blue);
-  box-shadow: none;
-}

assets/styles/app.scss

@@ -317,45 +317,6 @@ footer #footer-resources svg {
   font-size: 21px;
 }
 
-/* Forms
-   ------------------------------------------------------------------------- */
-.form-control {
-  color: var(--gray-700);
-  height: 45px;
-}
-
-.form-group .form-control:focus {
-  color: var(--text-color);
-}
-
-.form-group.has-error .form-control {
-  border-color: var(--red);
-}
-
-.form-group.has-error .control-label {
-  color: var(--red);
-}
-
-.form-group.has-error .help-block {
-  background-color: var(--red);
-  color: var(--white);
-  font-size: 15px;
-  padding: 1em
-}
-
-.form-group.has-error .help-block ul,
-.form-group.has-error .help-block li {
-  margin-bottom: 0
-}
-
-.form-group.has-error .help-block li + li {
-  margin-top: 0.5em;
-}
-
-textarea {
-  max-width: 100%
-}
-
 /* Page: 'Technical Requirements Checker'
    ------------------------------------------------------------------------- */
 body#requirements_checker header h1 {