maint: drop syncing with Automake::Struct

maint: drop syncing with Automake::Struct

The module Automake::Struct has been removed in automake master
branch (with yesterday's commit v1.11-2055-g74a7f49 "maint: drop
'Automake::Struct' module"): since Automake now requires Perl 5.6,
that module has become obsolete, being basically just a backport
of Perl 5.6's 'Class::Struct' to Perl 5.5.  With this change, we
follow suite in Autoconf, which syncs some of its internal modules
with Automake.

* lib/Autom4te/Struct.pm: Delete.
* lib/Autom4te/Makefile.am (dist_perllib_DATA): Don't list it
anymore.
* cfg.mk: Don't sync it with the Automake repository anymore.
* lib/Autom4te/Request.pm: Use 'Class::Struct' instead of
'Autom4te::Struct'.