README: remove msg
README: remove msg

This isn't used anymore. Grep for `msg` and the only instance seen is
rail-fence-cipher, and it's used as an input in that case,o

Evidence of our past intent to remove the `msg` key:

* #163
* [2dee540593776ba138e89670b1aaf611db631da5][cmt1]
* [1483fe9fafcc74f9e09b518cf0de9065047d3eb7][cmt2]

[cmt1]: https://github.com/exercism/x-common/commit/2dee540593776ba138e89670b1aaf611db631da5
[cmt2]: https://github.com/exercism/x-common/commit/1483fe9fafcc74f9e09b518cf0de9065047d3eb7

rpottsoh/x-common,petertseng/x-common,jmluy/x-common,rpottsoh/x-common,jmluy/x-common,exercism/x-common,Vankog/problem-specifications,petertseng/x-common,Vankog/problem-specifications,kgengler/x-common,kgengler/x-common,jmluy/x-common,ErikSchierboom/x-common,ErikSchierboom/x-common,exercism/x-common