Remove shared/math folder

The only remaining file was atanh, which is only used in math/atanh. Move it to
the math folder.

The folder itself has been cleared when support for Ruby 2.6 has been dropped.
References:
* commit 055d83c
* commit 653c06d

Author: herwinw

core/math/atanh_spec.rb

@@ -1,6 +1,6 @@
 require_relative '../../spec_helper'
 require_relative '../../fixtures/math/common'
-require_relative '../../shared/math/atanh'
+require_relative 'shared/atanh'
 
 describe "Math.atanh" do
   it_behaves_like :math_atanh_base, :atanh, Math