Multiple 'make uninstall' should not fail even for TEXINFOS.
* lib/am/texinfos.am (uninstall-info-am): Do not fail due to
install-info if the installed file does not exist (any more).
* tests/txinfo26.test: Issue multiple `make uninstall'.
* tests/instmany-mans.test: Likewise.
* tests/instmany-python.test: Likewise.
* tests/instmany.test: Likewise.
* tests/txinfo26.test: Likewise.
* NEWS: Update.