[Fix] RT-2544 'hide' button behavior in 'My Orders' in trade tab

Originally, both "My Orders" and "Orderbook" used ng-class, see
https://github.com/ripple/ripple-client/commit/139dac6859d40a22161b5a3edba68fc606d23478#diff-a5dc867b23059f00c00a1db4419375a3

Then later on @vhpoet switched the one in "Orderbook" to ng-hide
https://github.com/ripple/ripple-client/commit/429b0f3f0c0d3de0e84384567f91fc4946f1a85b#diff-a5dc867b23059f00c00a1db4419375a3