Further remove references to concurrent

Author: leeyi45

README.md

@@ -64,7 +64,7 @@ You can set additional options:
 Usage: js-slang [PROGRAM_STRING] [OPTION]
 
   -c, --chapter=CHAPTER set the Source chapter number (i.e., 1-4)                                                              (default: 1)
-  -v, --variant=VARIANT set the Source variant (i.e., default, interpreter, substituter, typed, concurrent, wasm) (default: default)
+  -v, --variant=VARIANT set the Source variant (i.e., default, interpreter, substituter, typed, wasm) (default: default)
   -h, --help            display this help
   -e, --eval            don't show REPL, only display output of evaluation
 ```

scripts/autocomplete.mjs

@@ -18,7 +18,6 @@ const TARGETS = [
   "source_2",
   "source_2_typed",
   "source_3",
-  "source_3_concurrent",
   "source_3_typed",
   "source_4",
   "source_4_typed",