Fix translations in non standard prefix

This drops a lot of code at the same time. The code in
__init__ was:

* Using a SUGAR_PREFIX environment variable that is not
defined anywhere
* Hardcoding prefix to /usr
* Setting up the sugar-base domain which doesn't exist anymore

The i18n module code was overcomplex because it was thought
for language packs which we don't support anymore, it was
not handling correctly locales with a country prefix, it
was hard coding the python prefix and it was not sorting
the directories as it intended too anyway.

The logic is very simple now. Use the locale directory in
the same prefix sugar-toolkit-gtk3 was installed, unless
SUGAR_LOCALEDIR is defined (for self contained bundles).

src/sugar3/activity/i18n.py

@@ -108,53 +108,3 @@ def pgettext(context, message):
     if '\x04' in translation:
         return message
     return translation
-
-
-def get_locale_path(bundle_id):
-    """ Returns the locale path, which is the directory where the preferred
-        MO file is located.
-
-        The preferred MO file is the one with the latest translation.
-
-        @type   bundle_id:      string
-        @param  bundle_id:      The bundle id of the activity in question
-        @rtype:                 string
-        @return:                the preferred locale path or None in the
-                                case of an error
-    """
-
-    # Note: We pre-assign weights to the directories so that if no translations
-    # exist, the appropriate fallbacks (eg: bn for bn_BD) can be loaded
-    # The directory with the highest weight is returned, and if a MO file is
-    # found, the weight of the directory is set to the MO's modification time
-    # (as described in the MO header, and _not_ the filesystem mtime)
-
-    candidate_dirs = {}
-
-    if 'SUGAR_LOCALEDIR' in os.environ:
-        candidate_dirs[os.environ['SUGAR_LOCALEDIR']] = 2
-
-    candidate_dirs[os.path.join(sys.prefix, 'share', 'locale')] = 0
-
-    default_locale = locale.getdefaultlocale()[0]
-    if not default_locale:
-        return None
-
-    for candidate_dir in candidate_dirs.keys():
-        if os.path.exists(candidate_dir):
-            full_path = os.path.join(candidate_dir,
-                                     default_locale, 'LC_MESSAGES',
-                                     bundle_id + '.mo')
-            if os.path.exists(full_path):
-                try:
-                    candidate_dirs[candidate_dir] = \
-                        _extract_modification_time(full_path)
-                except (IOError, ValueError):
-                    # The mo file is damaged or has not been initialized
-                    # Set lowest priority
-                    candidate_dirs[candidate_dir] = -1
-
-    available_paths = sorted(candidate_dirs.iteritems(), key=lambda (k, v):
-                            (v, k), reverse=True)
-    preferred_path = available_paths[0][0]
-    return preferred_path