There is nothing in the law pertaining to the racquet specific to removing -or- altering the 'buttcap' - it is section 4.2 in the laws.
To translate in common phrasing of the laws, length of the racquet is max 68cm and width of the head upto 23cm , and the length of the stringed area (22cm wide, 28cm long, max 33cm).
So butt cap away.
