Revert "Remove spec file before building"

This reverts commit af60443.

The issue that led to introducing it was never reproduced. I tried to
repro with this patch and it still works just fine. Since this removal
is getting in the middle for some race conditions I'm facing, I'm
reverting the patch.

lib/rubygems/installer.rb

@@ -293,8 +293,6 @@ def spec
   def install
     pre_install_checks
 
-    FileUtils.rm_f File.join gem_home, 'specifications', spec.spec_name
-
     run_pre_install_hooks
 
     # Set loaded_from to ensure extension_dir is correct

test/rubygems/test_gem_installer.rb

@@ -948,21 +948,18 @@ def test_install
 
     Gem.pre_install do
       assert_path_not_exist cache_file, 'cache file must not exist yet'
-      assert_path_not_exist spec_file,  'spec file must not exist yet'
       true
     end
 
     Gem.post_build do
       assert_path_exist gemdir, 'gem install dir must exist'
       assert_path_exist rakefile, 'gem executable must exist'
       assert_path_not_exist stub_exe, 'gem executable must not exist'
-      assert_path_not_exist spec_file, 'spec file must not exist yet'
       true
     end
 
     Gem.post_install do
       assert_path_exist cache_file, 'cache file must exist'
-      assert_path_exist spec_file,  'spec file must exist'
     end
 
     @newspec = nil