Remove unused loader argument from Plugin initializer

[bdewater]

I've been working on adding Puma instrumentation to our internal metrics gem. By reading the plugin code I got it to work in a way we want (no configuration required to enable the plugin from the gem) but I noticed I had to pass a slightly confusing loader argument when initializing the plugin.

In the current version of the gem, strangely enough the Plugin class is passed as loader, while it happens inside the PluginLoader class:

puma/lib/puma/plugin.rb

Line 13 in b646fc5

plugin = cls.new(Plugin)

this seems to have been an error made during refactoring in 33e0fa9 as the original implementation made more sense:

puma/lib/puma/plugin_loader.rb

Line 11 in 663666c

plugin = cls.new(self)

but even at that time, it seems the loader argument was never used, hence this PR removing it.

While technically part of the plugins API:

puma/docs/plugins.md

Lines 37 to 38 in 6bb070b

	Any public methods in `Puma::Plugin` are the public API that any plugin may
	use.

this does not affect the documented Puma::Plugin.create do ... end way of doing things.

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[nateberkopec]

Thx ❤️