Plugin template: Remove unnecessary code

[daipom]

Which issue(s) this PR fixes:
None

What this PR does / why we need it:
This code in test/helper.rb is unnecessary, so we should remove it.

$LOAD_PATH.unshift(File.expand_path("../../", __FILE__))

File.expand_path("../../", __FILE__) returns the repository's top directory path.
So, this enables using relative paths from the top for require, such as require test/helper.

However, we don't use this.
We use the following and use the relative paths from test/ or lib/.

fluentd/templates/new_gem/Rakefile

Lines 6 to 7 in 37858d3

	Rake::TestTask.new(:test) do |t|
	t.libs.push("lib", "test")

fluentd/templates/new_gem/test/plugin/test_input.rb.erb

Lines 1 to 2 in 37858d3

	require "helper"
	require "fluent/plugin/<%= plugin_filename %>"

So I think it's unnecessary as template content.

Docs Changes:

• Plugin development: Update test helper template fluentd-docs-gitbook#449

Release Note:
Same as the title.

[ashie]

Discussed in fluent-plugins-nursery/fluent-plugin-flowcounter-simple#7 (comment)

[daipom]

Thanks for your review!