FormLayout: remove hack

• usage of let caused an obscure bug caused by Qt. We can now remove the hack. teams/plasma-mobile/issues#88 (closed)

Tested with the app that originally showed the bug (keysmith) and no crash occurred. To test I used this command.

kdesrc-build kirigami --no-src && source ~/kde/build/kirigami/prefix.sh.fish && kdesrc-build keysmith --no-src && kdesrc-run keysmith