How to delete addition of rest

The + sign shows that the measure's duration is more than the standard duration based on the time signature.
(A - sign would show that the measure's duration is less than the standard duration.)

You probably got yourself into Insert mode while inputting the notes. See the Handbook topic Note input modes:
https://musescore.org/en/handbook/note-input#note-input-modes

To fix it:
- right-click a blank area in the measure
- select Measure Properties
- adjust the Actual measure duration back to the correct number for the time signature
- be aware this will truncate notes at the end of the measure if you reduce the duration