how to remove all accidentals

Please post the score. I think you may have done something wrong or had Concert Pitch on because transposition normally does not leave rogue accidentals.

You wrote:
...there are a bunch of accidental marks left over by the previous key (D major to G major; all c's are sharp).

Simply dragging a G major signature onto a D major score only changes the key signature. The C# notes don't change pitch. They were C# in D major - but without the accidental showing, as it's already built into the D major key signature. Then when the G major signature is dragged to replace the D major signature, the C notes remain sharp - just as they were in D major - only now, in G major, the C's must display a sharp to remain C#.

To transpose, to G major, music that has already been entered in D major, select it and use Tools -> Transpose.
See:
https://musescore.org/en/handbook/transposition#automatic-transpose
This moves all the notes to the new key, maintaining the same intervals between adjacent notes, so the 'tune sounds the same' - only pitched higher (or lower, as the case may be).

But if transpose from the answer above isn't what you're looking for now and simply wish to correct these "after the fact":

1. Right-click a single C#: Select → More…
2. Select either same pitch (that exact note) or likely same notename (same notename in all octaves) and press OK
3. Press the down arrow on your keyboard once ↓