Partially revert "[NFC][msan] Split ThreadStart and Init"

I don't know if removing "if (!start_routine_)" from ThreadStart
is NFC.

This reverts commit b3267bb3afd3c70a415ae81c855462affae92f6b.

GitOrigin-RevId: a5b06ad39a9babfeababc537b85bfdbcf127290d
1 file changed