Do not set MULTILINE_COMMENT_PREFIXES before calling update-copyright

This was a local hack to work around a limitation in update-copyright.
But the limitation is now OBE, because the copyright statements should
now always fit on a single line, thanks to the new policy of using
one single year range in the copyright notice.

gdb/ChangeLog:

        * copyright.py (MULTILINE_COMMENT_PREFIXES): Delete.
        (update_files): Do not set MULTILINE_COMMENT_PREFIXES
        environment variable before calling update-copyright.

gdb/copyright.py

@@ -35,14 +35,6 @@ import os
 import os.path
 import subprocess
 
-# A list of prefixes that start a multi-line comment.  These prefixes
-# should not be repeatead when wraping long lines.
-MULTILINE_COMMENT_PREFIXES = (
-    '/*',    # C/C++
-    '<!--',  # XML
-    '{',     # Pascal
-)
-
 
 def get_update_list():
     """Return the list of files to update.
@@ -80,10 +72,6 @@ def update_files(update_list):
 
     We use gnulib's update-copyright script for that.
     """
-    # Tell the update-copyright script that we do not want it to
-    # repeat the prefixes in MULTILINE_COMMENT_PREFIXES.
-    os.environ['MULTILINE_COMMENT_PREFIXES'] = \
-        '\n'.join(MULTILINE_COMMENT_PREFIXES)
     # We want to use year intervals in the copyright notices, and
     # all years should be collapsed to one single year interval,
     # even if there are "holes" in the list of years found in the