Fix TODO for compute coef_train for all training images

[tsainez]

The lab11.m file contained a TODO comment indicating that the training coefficients coef_train needed to be computed for all training images. The placeholder line %coef_train = zeros(k, n_train); was commented out, and immediately followed by coef_train = U'*Y; which mathematically computes the correct k x n_train coefficient matrix. This commit simply removes the obsolete TODO comment and commented out zero-initialization line.

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PR created automatically by Jules for task 13579861664170823562 started by @tsainez

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