Remove unused PluginClassRemapper

This was the very first cut at plugin class remapping; no longer
used (in favor of the comprehensive SpecialSource symbol remapper).
Remove old file-based plugin remapper

Removes the deprecated file-based plugin class remapper based on SrgTools.
This remapper has been off by default for about a month now, in favor of
the new SpecialSource in-memory remapper added in build 27. Removing the
old remapper to avoid confusion.

If you still want SrgTools the instructions can be found at:

https://github.com/MinecraftPortCentral/MCPC-Plus/tree/5210aa0c613f3a1bdccaadc8af9f1b253e871da3/plugins

but SpecialSource is preferrable, and includes a much more thorough
remapping (inheritance, reflection, configurability, and more) compatible
with more plugins. That is, the bukkit.yml configuration option remap-plugin-file
has been removed since its superseded by custom-class-loader (on by default).

If you're still using the old remapper (remap-plugin-file:true), please try the
new remapper (custom-class-loader:true) and report any problems. SpecialSource is
actively maintained; well-researched bug reports are welcome, especially on
any regressions from SrgTools (none are known at this time).