stbt lint: Remove unnecessary PYTHONPATH manipulation
stbt lint: Remove unnecessary PYTHONPATH manipulation

This shouldn't be necessary since `_stbt.pylint_plugint` is installed
onto the system python path since f7cfb1a (Install `_stbt` library into
global python directory).

stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester