Drop support for "fat gem" release

"fat gem" release has never actually been done in the past, and we have no plan to do it in the near future.
See GH#148 for the detailed discussion. ruby#148

Author: amatsuda

Rakefile

@@ -6,20 +6,7 @@ require "rake/extensiontask"
 require "rake/testtask"
 
 spec = eval(File.read('bigdecimal.gemspec'))
-Rake::ExtensionTask.new('bigdecimal', spec) do |ext|
-  ext.lib_dir = File.join(*['lib', ENV['FAT_DIR']].compact)
-  ext.cross_compile = true
-  ext.cross_platform = %w[x86-mingw32 x64-mingw32]
-  ext.cross_compiling do |s|
-    s.files.concat ["lib/2.3/bigdecimal.so", "lib/2.4/bigdecimal.so", "lib/2.5/bigdecimal.so"]
-  end
-end
-
-desc "Compile binaries for mingw platform using rake-compiler-dock"
-task 'build:mingw' do
-  require 'rake_compiler_dock'
-  RakeCompilerDock.sh "bundle && rake cross native gem RUBY_CC_VERSION=2.4.2:2.3.0:2.5.0"
-end
+Rake::ExtensionTask.new('bigdecimal', spec)
 
 Rake::TestTask.new do |t|
   t.libs << 'test/lib'

bigdecimal.gemspec

@@ -34,7 +34,6 @@ Gem::Specification.new do |s|
 
   s.add_development_dependency "rake", "~> 10.0"
   s.add_development_dependency "rake-compiler", ">= 0.9"
-  s.add_development_dependency "rake-compiler-dock", ">= 0.6.1"
   s.add_development_dependency "minitest", "< 5.0.0"
   s.add_development_dependency "pry"
 end

lib/bigdecimal.rb

@@ -1,5 +1 @@
-begin
-  require "#{RUBY_VERSION[/\d+\.\d+/]}/bigdecimal.so"
-rescue LoadError
-  require 'bigdecimal.so'
-end
+require 'bigdecimal.so'