Remove excessive `stbt run -v` underrun-handler debug
Remove excessive `stbt run -v` underrun-handler debug

This debug output is now available with `stbt run -vv` (two vees).

The debug when we actually stop & restart the pipeline is still visible
at `-v`, but the wording has been made more user-friendly.

wmanley/stb-tester,LewisHaley/stb-tester,wmanley/stb-tester,martynjarvis/stb-tester,stb-tester/stb-tester,martynjarvis/stb-tester,wmanley/stb-tester,LewisHaley/stb-tester,wmanley/stb-tester,martynjarvis/stb-tester,wmanley/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester