[ruby/irb] workspace.rb cleanup

(https://github.com/ruby/irb/pull/489)

* Remove unnecessary Binding#source_location check

`Binding#source_location` was added in 2.6, which is the minimum supported
version now. So this check is no longer necessary.

* Remove unused IRB.delete_caller

This method was added in the earliest version of IRB:

f47808999d

But it's not currently referenced by anything. We can verify this with a
org-wide search result:

https://github.com/search?q=org%3Aruby+delete_caller&type=code

lib/irb/workspace.rb

@@ -142,11 +142,7 @@ EOF
     end
 
     def code_around_binding
-      if @binding.respond_to?(:source_location)
-        file, pos = @binding.source_location
-      else
-        file, pos = @binding.eval('[__FILE__, __LINE__]')
-      end
+      file, pos = @binding.source_location
 
       if defined?(::SCRIPT_LINES__[file]) && lines = ::SCRIPT_LINES__[file]
         code = ::SCRIPT_LINES__[file].join('')
@@ -173,8 +169,5 @@ EOF
 
       "\nFrom: #{file} @ line #{pos + 1} :\n\n#{body}#{Color.clear}\n"
     end
-
-    def IRB.delete_caller
-    end
   end
 end