Thanks Mats,
Adding this to the \layout block removes the string numbers:
\context { \Voice
\remove New_fingering_engraver
}
"String_number_engraver" is apparently part of TabStaff.
Apparently it was easy for someone to keep string numbers, fretting hand,
and stroke hand numbers all in the same engraver called
"New_fingering_engraver". But globally removing string numbers also
clobbers the finger numbers. That's ok for my case but maybe in the future
3 different tranparent properties could be added to the
New_fingering_engraver to let you turn on/off fret hand, stroke hand, and
string numbers independently.