Clean up MAX_ABBREV_LENGTH

[erkyrath]

This is currently a compiled limit both for the Abbreviate directive and the abbreviation optimizer. Note that other abbreviation optimizers can produce arbitrarily long abbreviations, which Inform then chokes on. (It doesn't throw an error; it just truncates the Abbreviate directive.) See thread: https://intfiction.org/t/zabbrevmaker-0-10-released/66589/11

We haven't thought about this, because if a long chunk of text is repeated, the author can spot it and define a string constant. We typically rely on the abbreviation optimizer to find small annoying repeated strings like "You are" or "e fo". However, it can find longer strings.