What should a person who had a tooth filling while junub do? Should he/she have the filling taken off of his/her mouth?
No, you should not be taking them off. It is not easy for you to have your fillings taken off. Whereas Allah commands as follows in the verses related to -wudu- ablution and ghusl:
“Allah does not desire to put on you any difficulty.” (Al-Ma’ida/ The Feast 5:6)
In a different verse:
“He has not laid upon you any difficulty in religion.” (Al-Hajj/ The Pilgrimage 22:78)
Besides, Shafii and Maliki madhabs count the inside of mount and nose as inner side of body, not the outer. Therefore, washing the mouth and nose is not obligatory -fard-, but sunnah according to them. You can also behave accordingly.