Drop rbs require in rdoc-srcdir (#16983)

This is currently blocking Ruby's doc generation:
https://github.com/ruby/actions/actions/runs/25895616458/job/76107951475

We'll need to make rbs work for the process so we can adopt the rbs-integrated
RDoc. But for now let's drop it to unblock doc generation.

tool/rdoc-srcdir

@@ -2,7 +2,6 @@
 
 require 'rubygems'
 require 'rdoc/rdoc'
-require 'rbs'
 
 # Make only the output directory relative to the invoked directory.
 invoked = Dir.pwd