fix(discriminator): apply built-in plugins to discriminator schema even if mergeHooks and mergePlugins are both false

[vkarpov15]

Fix #12696

Summary

#12696 correctly points out that if both mergeHooks and mergePlugins are false, then we don't apply built-in plugins. Which is a problem because then we don't call save hooks on subdocs when parent hooks are saved, we don't call validate before save, etc. We use plugins for some of Mongoose's core functionality for dogfooding purposes, so we need to apply our built-in plugins even if the user doesn't want to apply user-defined global plugins to discriminator schemas.

Examples

[hasezoey]

LGTM

added a suggestions to fix a lint warning

also why was clearValidating deleted, was it unused?

just to clarify, applyBuiltinPlugins is different from using mongoose._applyPlugins because that function would also apply user-defined global plugins, right?

maybe also consider replacing

mongoose/lib/index.js

Lines 111 to 118 in 2fd0d29

	value: [
	[saveSubdocs, { deduplicate: true }],
	[validateBeforeSave, { deduplicate: true }],
	[shardingPlugin, { deduplicate: true }],
	[removeSubdocs, { deduplicate: true }],
	[trackTransaction, { deduplicate: true }]
	]
	});

with Object.values(builtinPlugins).map(v => [v, { deduplicate: true }]) to have built-in plugins defined in one place instead of multiple