By providing these properties to your button you can completely remove

BORDER of your button control. The flat-style button property is the
 
best property to set when you are working with custom button control.

you have to define this property into  Page load method.

Btnsubmit.TabStop = false;
Btnsubmit.FlatStyle = FlatStyle.Flat;
Btnsubmit.FlatAppearance.BorderSize = 0;
Btnsubmit.FlatAppearance.BorderColor = Color.FromArgb(0, 255, 255, 255);