DEV: Remove isTablet capability (#30799)

Reverts 02113fc. 

This is an imperfect detection of tablets and more generally, we want to
move away from detecting specific devices. THere's a broader effort to
remove mobile/desktop detection and rely instead on viewport-width-based
patterns and feature detection (touch, hover, etc.). See
https://github.com/discourse/discourse/pull/30642

To reach the same results in CSS/jS, we can use the `touch` and `hover`
media queries.

In CSS, something like: 

```
@media (hover: none) {
  // hover non excludes touchscreen desktops
  .discourse-touch {
    // we detect touch capability on the JS side, a bit of a belts and suspenders approach
  }
}
```

And in JS: 

```
this.capabilities.touch` plus `window.matchMedia("(hover: none)").matches
```

app/assets/javascripts/discourse/app/instance-initializers/sniff-capabilities.js

@@ -16,9 +16,5 @@ export default {
     if (caps.isIOS) {
       html.classList.add("ios-device");
     }
-
-    if (caps.isTablet) {
-      html.classList.add("tablet-device");
-    }
   },
 };

app/assets/javascripts/discourse/app/services/capabilities.js

@@ -9,15 +9,6 @@ class Capabilities {
   isAndroid = ua.includes("Android");
   isWinphone = ua.includes("Windows Phone");
   isIpadOS = ua.includes("Mac OS") && !/iPhone|iPod/.test(ua) && this.touch;
-  isTabletScreen =
-    this.touch &&
-    ((window.innerWidth >= 600 && window.innerWidth <= 1280) ||
-      (window.innerHeight >= 600 && window.innerHeight <= 1280));
-  isTablet =
-    this.isTabletScreen ||
-    this.isIpadOS ||
-    /iPad|Android(?!.*Mobile)|Tablet/.test(ua);
-
   isIOS = (/iPhone|iPod/.test(ua) || this.isIpadOS) && !window.MSStream;
   isApple =
     APPLE_NAVIGATOR_PLATFORMS.test(navigator.platform) ||