Remove useless monkey-patch of zc.buildout

SlapOS buildout does not have this method anymore.

About upstream buildout, we don't know because we don't use,
but signatures are less strict so maybe the patch was also useless.

slapos/rebootstrap/__init__.py

@@ -26,9 +26,6 @@ class extension(object):
     self.wanted_python = buildout[self.python_section]['executable']
     if sys.executable != self.wanted_python:
       self.hook('_setup_directories')
-    elif buildout._parts:
-      self._frozen = frozenset(buildout._parts)
-      self.hook('_compute_part_signatures')
 
   def hook(self, attr):
     buildout = self.buildout
@@ -91,17 +88,6 @@ Buildout will be restarted automatically to have this change applied.
     shutil.copy(new_bin, installed)
     os.execve(self.wanted_python, [self.wanted_python] + sys.argv, self.environ)
 
-  def _compute_part_signatures(self, install_parts):
-    # Skip signature check for parts that were required to build the wanted
-    # Python. Signatures differ when switching to a different version.
-    buildout = self.buildout
-    buildout._compute_part_signatures(install_parts)
-    installed_part_options = buildout.installed_part_options
-    for part in self._frozen.intersection(install_parts):
-      if part in installed_part_options:
-        buildout[part]['__buildout_signature__'] = \
-          installed_part_options[part]['__buildout_signature__']
-
 
 class Cache(easy_install.Installer):