Removed unnecessary warning messages

[gabrielenava]

Following #408, I removed two unnecessary warning messages.

I changed the condition of the if statement to avoid to have empty else statements in the logic. I compiled it on Ubuntu 18.04 and tested and it seems ok to me.

I targeted devel branch for the PR. If there is any changelog to update, I will update it.

[CLAassistant]

All committers have signed the CLA.

[gabrielenava]

cc @nunoguedelha @traversaro @S-Dafarra @CarlottaSartore

[traversaro]

Thanks @gabrielenava , please update the CHANGELOG in https://github.com/gabrielenava/idyntree/blob/removedUnnecessaryWarnings/CHANGELOG.md under Unreleased Major .

[gabrielenava]

Changelog updated under removed list.

[traversaro]

Thanks @gabrielenava !