[ruby/pathname] gemspec: Explicitly list 0 executables

This gem exposes no executables.

https://github.com/ruby/pathname/commit/c401d97d58

ext/pathname/pathname.gemspec

@@ -19,7 +19,7 @@ Gem::Specification.new do |spec|
     `git ls-files -z`.split("\x0").reject { |f| f.match(%r{^(test|spec|features)/}) }
   end
   spec.bindir        = "exe"
-  spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
+  spec.executables   = []
   spec.require_paths = ["lib"]
   spec.extensions    = %w[ext/pathname/extconf.rb]
 end