Migrate PID file management to core

[pgayvallet]

The PID file management needs to be moved to core, probably inside the EnvironmentService, and the pid mixin needs to be removed.

kibana/src/legacy/server/pid/index.js

Lines 25 to 30 in bf04235

	export default Bluebird.method(function (kbnServer, server, config) {
	const path = config.get('pid.file');
	if (!path) return;
	const pid = String(process.pid);

kibana/src/legacy/server/kbn_server.js

Lines 96 to 97 in bcaffba

	// writes pid file
	pidMixin,

[elasticmachine]

Pinging @elastic/kibana-platform (Team:Platform)