Remove python version properties

Python2 has been deleted, only python3 exists in the build now.
Cleanup version-specific python properties.

Also remove any embedded_launcher: true lines, as that is the
default now.

Bug: 203436762
Test: Presubmits
Flag: EXEMPT PURE_REFACTOR
Change-Id: I5405f2d6a1cefb10f6482a97c04f795406b18f48