Stop manually creating a `src` dir in the Pip dependencies layer

[edmorley]

Since Pip will create the directory itself if needed (which is only when there are editable mode VCS dependencies being installed).

This behaviour is already tested, via: https://github.com/heroku/buildpacks-python/blob/ff51b31c65cbd1932fc4cdc585c60942efcfe836/tests/pip_test.rs#L197-L216

GUS-W-16390558.