Ian Wienand 34cc121b98 Remove the -plain job variants
With the merge of I76befd9e2a054798e8e36500b723870f7c7ae328 the main
images have dropped pip-and-virtualenv.  Remove the plain variant
testing.

Change-Id: If442e15a0264821207725c1cc271b3ff646b0720
2020-06-17 11:47:54 +10:00
..