[docs] Remove ad information icon

[oliviertassinari]

Follow up on #16380 (comment).

Remove the information icon. It makes it "cleaner". Sebastian was legitimately confused by the display of an i, he is probably not the only one. It can look like a visual bug. My perspective was biased because I originally added it.

[mui-pr-bot]

Details of bundle changes.

Comparing: 7ae741f...7f088bc

bundle	parsed diff	gzip diff	prev parsed	current parsed	prev gzip	current gzip
@material-ui/core	0.00%	0.00%	319,993	319,993	88,291	88,291
@material-ui/core/Paper	0.00%	0.00%	68,292	68,292	20,371	20,371
@material-ui/core/Paper.esm	0.00%	0.00%	61,574	61,574	19,162	19,162
@material-ui/core/Popper	0.00%	0.00%	28,945	28,945	10,401	10,401
@material-ui/core/Textarea	0.00%	0.00%	5,507	5,507	2,362	2,362
@material-ui/core/TrapFocus	0.00%	0.00%	3,753	3,753	1,579	1,579
@material-ui/core/styles/createMuiTheme	0.00%	0.00%	16,010	16,010	5,790	5,790
@material-ui/core/useMediaQuery	0.00%	0.00%	2,597	2,597	1,103	1,103
@material-ui/lab	0.00%	0.00%	140,286	140,286	43,459	43,459
@material-ui/styles	0.00%	0.00%	51,699	51,699	15,346	15,346
@material-ui/system	0.00%	0.00%	15,420	15,420	4,390	4,390
Button	0.00%	0.00%	84,443	84,443	25,723	25,723
Modal	0.00%	0.00%	14,534	14,534	5,087	5,087
Portal	0.00%	0.00%	3,473	3,473	1,573	1,573
Slider	0.00%	0.00%	75,108	75,108	23,306	23,306
colorManipulator	0.00%	0.00%	3,904	3,904	1,544	1,544
docs.landing	0.00%	0.00%	54,338	54,338	13,762	13,762
docs.main	-0.11%	-0.11%	647,688	646,952	204,107	203,880
packages/material-ui/build/umd/material-ui.production.min.js	0.00%	0.00%	292,977	292,977	84,077	84,077

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