Remove has_rdoc from gemspec

Gem specification attribute #has_rdoc is deprecated and ignored.
According to deprecation warning, it may be removed "on or after
2018-12-01" with no replacement.

This attribute used to describe whether RDoc can be generated for given
gem or not.  According to RubyGems v1.3.3 release notes, "RDoc is now
generated regardless of Gem::Specification#has_rdoc?".

See:
- https://blog.rubygems.org/2009/05/04/1.3.3-released.html

gpgme.gemspec

@@ -7,7 +7,6 @@ Gem::Specification.new do |s|
   s.extensions        = ['ext/gpgme/extconf.rb']
   s.files             = Dir['{lib,ext,test,examples}/**/*'] +
                         Dir['ports/archives/*']
-  s.has_rdoc          = true
   s.rubyforge_project = 'ruby-gpgme'
   s.homepage          = 'http://github.com/ueno/ruby-gpgme'
   s.license           = 'LGPL-2.1+'