Remove method application via operator ##. (#5844)

* Remove method application via operator `##`.

The `.ml` syntax `x ## foo 3` does not exist in `.res`.
Only `x ## foo` exists, and is written `x["foo"]`.
Clean up compiler processing for those method applications.

* Remove test that uses `##` method application which no longer exists.

* Update CHANGELOG.md

Author: cristianoc

CHANGELOG.md

@@ -50,6 +50,7 @@ These are only breaking changes for unformatted code.
 - PPX V4: allow uncurried `make` function and treat it like a curried one [#5802](https://github.com/rescript-lang/rescript-compiler/pull/5802) [#5808](https://github.com/rescript-lang/rescript-compiler/pull/5808) [#5812](https://github.com/rescript-lang/rescript-compiler/pull/5812)
 - Remove processing of objects expressions, which don't exist in `.res` syntax (`Pexp_object`) https://github.com/rescript-lang/rescript-compiler/pull/5841
 - Remove class type processing from compiler ppx https://github.com/rescript-lang/rescript-compiler/pull/5842
+- Remove method application via operator `##`, which does not exist in `.res` syntax https://github.com/rescript-lang/rescript-compiler/pull/5844
 
 # 10.1.0-rc.6