You should be able to remove these by hooking into the mce_css
filter in your functions.php
file:
add_filter("mce_css", "my_remove_mce_css");
function my_remove_mce_css($stylesheets){
return "";
}
I haven’t tested this but it should work. You might want to echo
out the value of $stylesheets
before you return nothing just to see what else is coming through – it might be that you want to retain some of the stylesheets, in which case you could remove the ones you don’t want with str_replace
.
You can read more about this filter at https://codex.wordpress.org/Plugin_API/Filter_Reference/mce_css