HPCC-8977 Clean Warning in mime.cpp

This fix removes the check of 'if (firstPartLength >=0)' since it should
never be negative.

Signed-off-by: Kevin Wang <kevin.wang@lexisnexis.com>
Merge pull request #4130 from wangkx/h8977

HPCC-8977 Clean Warning in mime.cpp

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>