Remove un-used and broken TriangleSAHPred<T>& operator= method

[Tyler799]

This method was a problem previously, but GCC14 notices that it cannot work given that vertex_stride_bytes_ is const.

It's not used, so just remove it.

[syoyo]

operator= may be used in some user code. Adding 'mutable' to vertex_stride_bytes_ would be a recommend way for the fix.

[syoyo]

I am changing my mind.
copy-assignment operator for class having const variables are troublesome and probably no one actually uses copy-assignment operator of TriangleSAHPred class.

https://stackoverflow.com/questions/3658854/does-it-make-sense-to-implement-the-copy-assignment-operator-in-a-class-with-all

So merged PR. Thanks!