Remove the use of deallog from step-3.

[bangerth]

Among the usual canon of tutorial programs students will see early on (say, until step-22, step-3 is alone in using deallog. It uses that for automatically outputting information about solver convergence. That could easily be replaced by a hand-written statement, and would avoid the need to explain a bunch of things in that program.

[bangerth]

In fact, we can just take what we already do in step-4.