Fix temporary string descriptor leak when ignoring string function results

bug from 6490c312

Only fb_StrDelete() was being used to delete ignored string results, but
that actually doesn't free the temporary descriptors used for string
function results, but only the string data. The descriptors were leaked
and can easily be exhausted - the rtlib only has 256.

Solution: call fb_hStrDelTemp() instead, as done in lots of rtlib code.
rtlStrDelete(): Clean up unnecessary dtype checks