refactor(env): remove unused mutex

This was needed when TUI was a thread.
lua code uses os_getenv only on the main thread.

src/nvim/main.c

@@ -176,7 +176,6 @@ bool event_teardown(void)
 /// Needed for unit tests. Must be called after `time_init()`.
 void early_init(mparm_T *paramp)
 {
-  env_init();
   estack_init();
   cmdline_init();
   eval_init();          // init global variables