test-stbt-auto-selftest.sh: Remove --with-experimental flag
test-stbt-auto-selftest.sh: Remove --with-experimental flag

The `stbt auto-selftest` command is no longer experimental since
9386787.

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