Use automake support for texi to generate version info in the manual

Automake contains some support for automagically adding version
information to a texi manual (see
https://www.gnu.org/software/automake/manual/automake.html#Texinfo).

> If the .texi file @includes version.texi, then that file will be
> automatically generated

This patch gets rid of the texi.texi.in and instead uses the standard
method to inject version information in the manual.

This fixes https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=870634

configure.ac

@@ -278,7 +278,6 @@ lib/parser/Makefile
 lib/tests/Makefile
 mu4e/Makefile
 mu4e/mu4e-meta.el
-mu4e/texi.texi
 guile/Makefile
 guile/texi.texi
 guile/mu/Makefile