Remove Projection and Operator

[hedaoyuan]

Paddle 中跟Layer相似的概念还有Projection和Operator，引入Function #892 的目的是把Computation相关的用统一的形式表示，Layer的计算实现上变成是对一些Function的调用，所以应该是可以去掉Projection和Operator。

/**
 * A projection takes one Argument as input, calculate the result and add it
 * to output Argument.
 */
class Projection

/**
 * Operator like Projection, but takes more than one Arguments as input.
 * @note: Operator can't have parameters.
 */
class Operator

[gongweibao]

Closing this issue due to inactivity, feel free to reopen it.