R1:c125ab7a42bf

Remove "metamta.*.subject-prefix" options

Summary:
In ~2012, the first of these options was added because someone who hates dogs and works at Asana also hated `[Differential]` in the subject line. The use case there was actually //removing// the text, not changing it, but I made the prefix editable since it seemed like slightly less of a one-off.

These options are among the dumbest and most useless config options we have and very rarely used, see T11760. A very small number of instances have configured…