Remove covariate expression from a function string
removeCovariate(funstring, varName, covariate, theta)a string giving the expression that needs to be modified
the variable to which the given string corresponds to in the model expression
the covariate expression that needs to be removed (from the appropriate place)
a list of names of the 'theta' parameters in the 'fit' object
returns the modified string with the covariate removed from the function string