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

source/blender/blenlib/intern/tempfile.cc

@@ -62,8 +62,6 @@ void BLI_temp_directory_path_get(char *tempdir, const size_t tempdir_maxncpy)
 #ifdef WIN32
       "TEMP",
 #else
-      /* Non standard (could be removed). */
-      "TMP",
       /* Posix standard. */
       "TMPDIR",
 #endif

source/creator/creator_args.cc

@@ -874,7 +874,6 @@ static void print_help(bArgs *ba, bool all)
     PRINT("  $TEMP                      Store temporary files here (MS-Windows).\n");
   }
   if (!defs.win32 || all) {
-    /* NOTE: while `TMP` checked, don't include here as it's non-standard & may be removed. */
     PRINT("  $TMPDIR                    Store temporary files here (UNIX Systems).\n");
   }
   PRINT(

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