[cleanup] Remove unused code paths (#2173)

Notes:

* `_windows_write_string`: Fixed in 3.6
  * https://bugs.python.org/issue1602
  * PEP: https://www.python.org/dev/peps/pep-0528

* Windows UTF-8 fix: Fixed in 3.3
  * https://bugs.python.org/issue13216

* `__loader__`: is always present in 3.3+
  * https://bugs.python.org/issue14646

* `workaround_optparse_bug9161`: Fixed in 2.7
  * https://bugs.python.org/issue9161

Authored by: fstirlitz

yt_dlp/__main__.py

@@ -2,8 +2,7 @@
 from __future__ import unicode_literals
 
 # Execute with
-# $ python yt_dlp/__main__.py (2.6+)
-# $ python -m yt_dlp          (2.7+)
+# $ python -m yt_dlp
 
 import sys