contrib: remove references to 2to3

The custom porting fixers are removed. A comment related to 2to3
has been removed from the import checker.

After this patch, no references to 2to3 remain.

contrib/import-checker.py

@@ -495,10 +495,6 b' def verify_stdlib_on_own_line(root):'
     """Given some python source, verify that stdlib imports are done
     in separate statements from relative local module imports.
 
-    Observing this limitation is important as it works around an
-    annoying lib2to3 bug in relative import rewrites:
-    http://bugs.python.org/issue19510.
-
     >>> list(verify_stdlib_on_own_line(ast.parse('import sys, foo')))
     [('mixed imports\\n   stdlib:    sys\\n   relative:  foo', 1)]
     >>> list(verify_stdlib_on_own_line(ast.parse('import sys, os')))

tests/test-check-py3-compat.t

@@ -6,9 +6,6 b''
   contrib/check-code.py not using absolute_import
   contrib/check-code.py requires print_function
   contrib/debugshell.py not using absolute_import
-  contrib/hgfixes/fix_bytes.py not using absolute_import
-  contrib/hgfixes/fix_bytesmod.py not using absolute_import
-  contrib/hgfixes/fix_leftover_imports.py not using absolute_import
   contrib/import-checker.py not using absolute_import
   contrib/import-checker.py requires print_function
   contrib/memory.py not using absolute_import