Remove client validation for loading certificate

[Elsie4ever]

This pr is the second part of addressing the enrollment group create/update with certificate issue. I removed the validation on the client side to make sure we are not over validating the certificate content, so that way the certificate validation now is fully handled by service call. The file extension check is removed and I kept the help text in place to give user more guidance on the certificate format

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[Elsie4ever]

[vilit1]

Add a history for the changes you did (specifically the fact that certificate files are no longer restricted by file extension)

[Elsie4ever]

@vilit1 history updated, please feel free to take another look, thanks!