Core: remove use of the environment variable TMP on Unix

This isn't a standard and seems only to be included for historic reasons.

Ref !144100

tests/python/bl_pyapi_bpy_app_tempdir.py

@@ -37,10 +37,6 @@ import tempfile
 is_win32 = os.name == "nt"
 TEMP_ENV = "TEMP" if is_win32 else "TMPDIR"
 
-# TODO: remove this. Since it's checked, it must be removed!
-if os.environ.get("TMP") is not None:
-    del os.environ["TMP"]
-
 
 def system_temp_set(path: str) -> None:
     os.environ[TEMP_ENV] = path