You remove a head by replacing it with Identity

Cos[a] /. Cos -> Identity

For doing this over lots of expressions:

list = {ArcTan[x], ArcTan[x], ArcTan[x], Cot[x], Cot[z], Cot[x], 
  ArcTan[z], ArcTanh[y], ArcTanh[x], Cot[y]};

list[[All, 0]] = Identity

You remove a head by replacing it with Identity

Cos[a] /. Cos -> Identity

For doing this over lots of expressions:

list = {ArcTan[x], ArcTan[x], ArcTan[x], Cot[x], Cot[z], Cot[x], 
  ArcTan[z], ArcTanh[y], ArcTanh[x], Cot[y]};

list[[All, 0]] = Identity

or

Identity @@@ list

etc