Remove blw variable in favour of calculating the value when needed

Remove blw variable in favour of calculating the value when needed

The purpose and reasoning behind the bar layout width (blw) variable
in dwm the way it is today may not be immediately obvious.

The use of the variable makes more sense when looking at commit
2ce37bc from 2009 where blw was initialised in the setup function
and it represented the maximum of all available layout symbols.

	for(blw = i = 0; LENGTH(layouts) > 1 && i < LENGTH(layouts); i++) {
		w = TEXTW(layouts[i].symbol);
		blw = MAX(blw, w);
	}

As such the layout symbol back then was fixed in size and both drawbar
and buttonpress depended on this variable.

The the way the blw variable is set today in drawbar means that it
merely caches the size of the layout symbol for the last bar drawn.

While unlikely to happen in practice it is possible that the last bar
drawn is not that of the currently selected monitor, which can result
in misaligned button clicks if there is a difference in layout symbol
width between monitors.