Revert "Fix #2072 (only resize a tab if the size changed since the last display)"

This reverts commit b46f0f5e266c321632738ca40839759486b47a7e.

Conflicts:
	src/tabs/muclisttab.py

Doing this made the unresized elements refresh in the old subwins,
causing glitches and weirdness. And anyway, the only problematic
element is the TextWin (rebuilding all the lines of a buffer is
expensive), but it already checks if the width changed.