"Unnecessary" accidentals - some questions/remarks.

They'll show up because your MusicXML requests for them to show up.

The fastest way to get rid of them is to select a range and then press up arrow followed by down arrow to respell everything and prefer flats, or the other way around to prefer sharps.

These accidentals - called "courtesy accidentals" are absolutely necessary to ensure proper reading, required by the conventions of music. It would be bad to remove them - this will all but guarantee people reading the score will make mistakes, continuing to play F# in the second measure, etc. Yes, technically, barlines cancel accidentals, but in real life, people don't usually catch that especially when sight reading, so it's expected you always explicitly cancel any accidental in the next measure. Some editors go further, and cancel accidentals even if it's not until several measures later.

A most excellent plugin exists for the purpose of automating the addition of these according to various different rules. It also allows you to remove them, but this should only be done as a plredue to then adding them back using different rules, or adding them manually - you should never actually leave a score without courtesy accidentals.