Add changelog entry for SDL2 update. NFC

Also remove the no-longer-needed `linker_setup` function.  SDL2 now
uses EM_JS_DEPS to achieve this.

Followup to #21337

Author: sbc100

ChangeLog.md

@@ -23,6 +23,7 @@ See docs/process.md for more on how version tagging works.
 
 3.1.54 - 02/15/24
 -----------------
+- SDL2 port updated from v2.24.2 to v2.26.0. (#21337)
 - The `DEMANGLE_SUPPORT` setting and the associated `demangle` function are
   now deprecated since Wasm stack traces always contain demangled symbols these
   days. (#21346)