Never add a vendored smmap directory to sys.path

This removes the logic that appended the git submodule directory
for smmap to sys.path under most circumstances when the version
of gitdb was not from PyPI. Now gitdb does not modify sys.path.

See gitpython-developers/GitPython#1717
and gitpython-developers/GitPython#1720 for
context. This change is roughly equivalent to the change to
GitPython, though as noted the behavior being eliminated is subtly
different here and there.

gitdb/__init__.py

@@ -4,34 +4,12 @@
 # the New BSD License: https://opensource.org/license/bsd-3-clause/
 """Initialize the object database module"""
 
-import sys
-import os
-
-#{ Initialization
-
-
-def _init_externals():
-    """Initialize external projects by putting them into the path"""
-    if 'PYOXIDIZER' not in os.environ:
-        where = os.path.join(os.path.dirname(__file__), 'ext', 'smmap')
-        if os.path.exists(where):
-            sys.path.append(where)
-
-    import smmap
-    del smmap
-    # END handle imports
-
-#} END initialization
-
-_init_externals()
-
 __author__ = "Sebastian Thiel"
 __contact__ = "byronimo@gmail.com"
 __homepage__ = "https://github.com/gitpython-developers/gitdb"
 version_info = (4, 0, 11)
 __version__ = '.'.join(str(i) for i in version_info)
 
-
 # default imports
 from gitdb.base import *
 from gitdb.db import *