Consider dropping docs/transient.texi from source distribution

[manphiz]

docs/transient.texi is generated during make docs, and it's output depends on the Emacs version when running make. This has been a source of issue that is causing transient failing to build twice-in-a-row in Debian. Relevant build failure log below (see also the Debian bug[1] regarding double-build check in Debian):

dpkg-source: info: using source format '3.0 (quilt)'
dpkg-source: info: building elpa-transient using existing ./elpa-transient_0.8.3.orig.tar.gz
dpkg-source: info: local changes detected, the modified files are:
 elpa-transient-0.8.3/docs/transient.texi
dpkg-source: error: aborting due to unexpected upstream changes, see /tmp/elpa-transient_0.8.3-1.diff.gGn3M9
dpkg-source: hint: make sure the version in debian/changelog matches the unpacked source tree
dpkg-source: hint: you can integrate the local changes with dpkg-source --commit
dpkg-buildpackage: error: dpkg-source -b . subprocess returned exit status 2

As docs/transient.texi is generated using Emacs anyway, I wonder whether it can be considered to drop it from source tarball distribution and only ship docs/transient.org, which should be sufficient.

[1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1044428 (Note that the bug contains build log for an earlier version which reports error for other generated files, but the idea is the same.)