Reboot: Remove now-unnecessary input[type="search"] box-sizing override

Revert "fixes #17379: override search input box-sizing to match our global overrides"

This reverts commits 468a9d93 & 57998dc2.

https://github.com/necolas/normalize.css/pull/496 has made this override unnecessary.

[skip sauce]

scss/_reboot.scss

@@ -331,8 +331,6 @@ legend {
 }
 
 input[type="search"] {
-  // Undo Normalize's default here to match our global overrides.
-  box-sizing: inherit;
   // This overrides the extra rounded corners on search inputs in iOS so that our
   // `.form-control` class can properly style them. Note that this cannot simply
   // be added to `.form-control` as it's not specific enough. For details, see