Remove .btn-blue

deprecations.js

@@ -7,6 +7,7 @@ const versionDeprecations = {
   '16.0.0': [
     {
       selectors: [
+        '.btn-blue',
         '.markdown-body li',
         '.input-dark',
         '.input-dark:-ms-input-placeholder',

docs/content/support/v16-migration.mdx

@@ -13,6 +13,12 @@ Because colors differ in each color mode, it doesn't make sense to try to make a
 
 Most components don't need to be updated and should work without making changes. But some of the componets that use presentational class names now use functional class names.
 
+#### Buttons
+
+| `v15`       | `v16` |
+| ----------- | ----- |
+| `.btn-blue` | n/a   |
+
 #### Counters
 
 | [`v15`](https://primer.style/css/components/labels#counters) | `v16`                 |

src/buttons/button.scss

@@ -109,7 +109,6 @@
 
 // Primary button
 
-.btn-blue, // TODO: deprecate
 .btn-primary {
   color: var(--color-btn-primary-text);
   background-color: var(--color-btn-primary-bg);