There is too much burden attached to working with KeyEvenTranslations.tr so I rarely use it.
You can try an alternative: DevTools`:NotebookActions:
ResourceFunction["GitHubInstall"]["kubapod", "devtools"]
Needs @ "DevTools`";
NotebookActionsEnable[];
EditNotebookActions[];
add a cell with:
<| "Label" -> "ClearCell"
, "ShortKey" -> "c"
, "Action" :> FrontEndExecute[{
SelectionMove[InputNotebook[],All,CellContents]
, NotebookDelete @ InputNotebook[]
}]
|>
Press Save and Test.
And you are ready to go: Ctrl+, and then c should clear cell contents.

KeyEventTranslations.tr. Also mentioned in that answer is this link which shows how to modify that file the right way. All the information you need is available in the links I have provided and the code modification in the answer above. Try it and if you are still unable to get it to work, add a comment and I will try to help out. – Rohit Namjoshi Dec 20 '18 at 04:41