Hi,
There is no option to get rid of units being displayed after X-label and Y-label using Toolbox Preferences Editor. Replacing text using regular expression may help.
fig = figure;
opts3 = bodeoptions('cstprefs');
opts3.Title.String = 'Black diagram';
opts3.XLabel.String = 'Fase [°]';
opts3.YLabel.String = 'Versterking (db)';
opts3.grid = 'on';
nichols(g1, opts3)
labels = findall(fig,'Type','Text');
label = labels(strncmp(get(labels,'String'),'Fase [°]',8));
set(label,'String',regexprep(get(label,'String'),'(\(\w*)\)',''));
Refer to the following link for more information on regexprep:https://www.mathworks.com/help/matlab/matlab_prog/regular-expressions.html#btrug0n-1