[rubygems/rubygems] Simplify and remove some unused code

When `install_with_build_args` was added in
be96283985,
there were two versions of the method: the default version in the base class that still
used the locking `with_build_args`, and an override in the `Future`
class (for Rubygems 2.0 and up) that yielded without calling
`with_build_args`.

The `with_build_args` version of the method was removed in
8a5b71e3e8
while removing a bunch of the old Rubygems compatibility code.

This commit removes `with_build_args`, since it no longer appears to be
used (the build args are passed as a keyword argument to
`spec.source.install` instead, since
be96283985).

The commit also removes `install_with_build_args` and the conditional
around it, since the method wasn't doing anything different than
`install`, and it had a comment that was no longer accurate.

https://github.com/rubygems/rubygems/commit/ba543a60eb

lib/bundler/installer/gem_installer.rb

@@ -13,7 +13,7 @@ module Bundler
     end
 
     def install_from_spec
-      post_install_message = spec_settings ? install_with_settings : install
+      post_install_message = install
       Bundler.ui.debug "#{worker}:  #{spec.name} (#{spec.version}) from #{spec.loaded_from}"
       generate_executable_stubs
       return true, post_install_message
@@ -54,11 +54,6 @@ module Bundler
       spec.source.install(spec, :force => force, :ensure_builtin_gems_cached => standalone, :build_args => Array(spec_settings))
     end
 
-    def install_with_settings
-      # Build arguments are global, so this is mutexed
-      Bundler.rubygems.install_with_build_args([spec_settings]) { install }
-    end
-
     def out_of_space_message
       "#{install_error_message}\nYour disk is out of space. Free some space to be able to install your bundle."
     end

lib/bundler/rubygems_integration.rb

@@ -232,18 +232,6 @@ module Bundler
       EXT_LOCK
     end
 
-    def with_build_args(args)
-      ext_lock.synchronize do
-        old_args = build_args
-        begin
-          self.build_args = args
-          yield
-        ensure
-          self.build_args = old_args
-        end
-      end
-    end
-
     def spec_from_gem(path, policy = nil)
       require "rubygems/security"
       require_relative "psyched_yaml"
@@ -550,10 +538,6 @@ module Bundler
       Gem::REPOSITORY_SUBDIRECTORIES
     end
 
-    def install_with_build_args(args)
-      yield
-    end
-
     def path_separator
       Gem.path_separator
     end