Remove require in signal handler to avoid ThreadError

[pocke]

This PR fixes a ThreadError, which is #282.

Fix #282

Problem

IRB does require 'rdoc' in SHOW_DOC_DIALOG and it is called from a trap context. Then it causes the ThreadError.

Solution

Remove the require from the proc. It looks unnecessary because rdoc is required from the top-level of the same file.

irb/lib/irb/input-method.rb

Line 17 in 5f749c6

require 'rdoc'

[aycabta]

Thank you!