Refactor tesseract_env variable into the plugin

Removed all cases except one in api.py, which isn't worth solving because
it should be removed anyway.

This also fixes a logic error in the OMP_THREAD_LIMIT decision, api.py
did not use pass kwargs correctly so they never worked before.

src/ocrmypdf/cli.py

@@ -474,7 +474,6 @@ Online documentation is located at:
         action='store_true',
         help="Keep temporary files (helpful for debugging)",
     )
-    debugging.add_argument('--tesseract-env', type=str, help=argparse.SUPPRESS)
     return parser