Fix bugs in handling combined injections exposed by HEEx (#2652)

Fixes
https://linear.app/zed-industries/issue/Z-2481/heex-this-snippet-triggers-a-hard-crash

Release Notes:

- Fixed a crash that would sometimes occur when editing a HEEx file
([#1703](https://github.com/zed-industries/community/issues/1703)).

crates/zed/src/languages/heex/injections.scm

@@ -9,7 +9,5 @@
   (#set! combined)
 )
 
-; expressions live within HTML tags, and do not need to be combined
-;     <link href={ Routes.static_path(..) } />
 ((expression (expression_value) @content)
  (#set! language "elixir"))