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.

autogen.sh

@@ -7,7 +7,6 @@ test -f mu/mu.cc || {
     echo "*** Run this script from the top-level mu source directory"
     exit 1
 }
-
 BUILDDIR=build
 
 command -v meson 2> /dev/null
@@ -23,8 +22,5 @@ else
     meson ${BUILDDIR} $@
 fi
 
-# Add a Makefile with some useful target
-cp Makefile.meson Makefile
-
 echo "*** Now run 'ninja -C ${BUILDDIR}' to build mu"
 echo "*** Or check the Makefile for some useful targets"