Remove dead code related to InstallRequirement build directories

[chrahunt]

Going back a few years, InstallRequirement._ideal_build_dir has been a string. As a result, the second half of move_to_correct_build_directory couldn't've been executed, because it was asserting self._ideal_build_dir.path, which is not an attribute on strings.

See:

1. self._ideal_build_dir is only set in InstallRequirement.ensure_build_location(build_dir), to the value of the build_dir parameter
2. Calls to InstallRequirement.ensure_build_location(build_dir):
   1. From move_to_correct_build_directory itself, but we can leave that aside as it is after the invalid assertion in question
   2. From WheelBuilder.build with RequirementPreparer.build_dir which is set to a string here
   3. From InstallRequirement.ensure_has_source_dir which itself is only called from
      1. RequirementPreparer.prepare_linked_requirement here with self.build_dir (already established as a string)
      2. RequirementPreparer.prepare_editable_requirement here with self.src_dir, which is a string per
         1. construction with options.src_dir here, the value of which is only ever a string per the only locations it is set
            1. DownloadCommand.run
            2. WheelCommand.run
            3. InstallCommand.run

[xavfernandez]

Nice catch 👍

[pradyunsg]

Hah! I was gonna file a similar PR -- you beat me to it. :)