How to remove the item that is repeated?

Keys@Select[Counts[list], # == 1 &]

{m, n, p, r}

Also:

Select[list, Count[list, #] == 1 &]
Select[list, Counts[list][#] == 1 &]
Flatten[Cases[Split[Sort[list]], {_}]]
Flatten[DeleteCases[Split[Sort[list]], {_, __}]]
Flatten[Select[Split[Sort[list]], Length@# == 1 &]]
SequenceReplace[list, {OrderlessPatternSequence[Repeated[b_, {2, Infinity}], a_]} :> a]

First /@ Select[Tally@list, Last@# == 1 &]

{m, n, p, r}

or

First /@ Cases[Tally@list, {_, 1}]

{m, n, p, r}