clean out legacy/excess/unmaintained text files

this includes:

- unmaintained AUTHORS, CHANGELOG, INSTALL, TODO files
- for the info slave the MIT license statement has been converted to
spdx allowing us to remove the MIT copy in the directory
- for the mtp slave the GPL copy was duplicating the one in LICENSES and
there were two copies to begin with
- other files have been dropped if they are not required but outdated,
or outdated information has been removed so long as the remaining file
still has use
- COPYING.GPLv2 and COPYING.LGLv2.0 are dupes of files in LICENSES