Revert "Highlight super and this as keywords in JS/TS/TSX" (#26342)

Reverts zed-industries/zed#25135

This approach was not the best as explained in the response to the
original PR. Likely, the better approach is to create a newer specific
scope for these kinds of variables under the `@variable` prefix so that
themes can control these pseudo-keywords specifically

crates/languages/src/javascript/highlights.scm

@@ -62,8 +62,8 @@
 
 ; Literals
 
-(this) @keyword
-(super) @keyword
+(this) @variable.special
+(super) @variable.special
 
 [
   (null)