autotools: remove

Since 2008, autotools has served us well - thank you!

However, mu is now using meson build, and it's time to remove the
autotools support -- one build system is enough.

man/Makefile.am

@@ -1,38 +0,0 @@
-## Copyright (C) 2008-2020 Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>
-##
-## This program is free software; you can redistribute it and/or modify
-## it under the terms of the GNU General Public License as published by
-## the Free Software Foundation; either version 3 of the License, or
-## (at your option) any later version.
-##
-## This program is distributed in the hope that it will be useful,
-## but WITHOUT ANY WARRANTY; without even the implied warranty of
-## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-## GNU General Public License for more details.
-##
-## You should have received a copy of the GNU General Public License
-## along with this program; if not, write to the Free Software Foundation,
-## Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
-
-include $(top_srcdir)/gtest.mk
-
-dist_man_MANS =		\
-	mu-add.1	\
-	mu-bookmarks.5	\
-	mu-cfind.1	\
-	mu-easy.1	\
-	mu-extract.1	\
-	mu-fields.1     \
-	mu-find.1	\
-	mu-help.1	\
-	mu-index.1	\
-	mu-info.1	\
-	mu-init.1	\
-	mu-mkdir.1	\
-	mu-query.7	\
-	mu-remove.1	\
-	mu-server.1	\
-	mu-script.1	\
-	mu-verify.1     \
-	mu-view.1	\
-	mu.1