remove the ConjugateGroup method based on nice monom. (#6197)

I think that using nice monomorphisms in `ConjugateGroup` methods
is a bad idea:

When this works (that is, when the conjugating element can be mapped
under the nice monomorphism of the group)
then one first maps the group and the element under the nice monomorphism,
then conjugates in the image,
and then pulls back the result under the monomorphism.
The group which one gets this way (via `GroupByNiceMonomorphism`)
knows almost nothing except the nice monomorphism.

And in general this will not work because the conjugating element will not fit.

Due to the omission of the `ConjugateGroup` method,
we will lose the known `NiceMonomorphism` in the result of `ConjugateGroup`.
Eventually this can be "reinserted" by installing a new
`ConjugateGroup` method with requirement `IsGroup and HasNiceMonomorphism`
that creates a nice monomorphism of the conjugated group.
Do we want this?
(In certain cases, this will be not what one wants:
Perhaps we conjugate the group just in order to get a better behaved group,
and then setting a "nice monomorphism" in this group
that delegates tasks via the "ugly" original group is a bad idea.)

Author: ThomasBreuer

lib/grpnice.gi

@@ -352,14 +352,6 @@ local mon,cl,clg,c,i;
 end);
 
 
-#############################################################################
-##
-#M  ConjugateGroup( <G>, <g> )  . . . . . . . . . . . . . .  conjugate of <G>
-##
-GroupMethodByNiceMonomorphismCollElm( ConjugateGroup,
-    [ IsGroup and HasParent, IsMultiplicativeElementWithInverse ] );
-
-
 #############################################################################
 ##
 #M  Core( <G>, <U> )  . . . . . . . . . . . . . . . .  core of a <U> in a <G>