c2a: Only use postsubmit when configured

Previous logic would place tests in both presubmit and postsubmit if the
postsubmit_tests configuration was not specified. This is both
unnecessary and wastes resources because presubmit tests are also run in
postsubmit.

Bug: 236006683
Test: Regenerate all TEST_MAPPINGs, manually check postsubmit removal
Change-Id: Ie8ff0e351776711e07bf7646efd916d970254e9c
1 file changed