Drop unused gridstack specific styles

[cedric-anne]

Checklist before requesting a review

• I have read the CONTRIBUTING document.
• I have performed a self-review of my code.

Description

These styles seems to be unused. The grid is redefined in

glpi/js/modules/Dashboard/Dashboard.js

Lines 1194 to 1205 in 7349c9d

	for (let i = 0; i < this.cols; i++) {
	const left = i * width_percent;
	const width = (i+1) * width_percent;
	style+= this.elem_id+" .grid-stack > .grid-stack-item[gs-x='"+i+"'] { \
	left: "+left+"%; \
	} \
	"+this.elem_id+" .grid-stack > .grid-stack-item[gs-w='"+(i+1)+"'] { \
	min-width: "+width_percent+"%; \
	width: "+width+"%; \
	}";
	}