services/horizon: remove go-codegen (#2371)

### What
Remove use of the go-codegen tool for generating the action handlers.

### Why
Go-codegen is difficult to install because it is using the old [gb] build tool that is no longer maintained. We're using go-codegen to generate 4 lines of code for each action type. There are 15 types that we generate those 4 lines of code for, which means we're using this tool to generate only 60 lines of code. We can easily write these lines of code by hand and the effort to keep using go-codegen is much higher than that.

Additionally, @bartekn mentioned that this code will disappear when we move away from actions in the near future so there is even less value in us keeping the go-codegen code generation working.

[gb]: http://github.com/constabulary/gb

services/horizon/internal/action.go

@@ -22,10 +22,6 @@ import (
 
 // Action is the "base type" for all actions in horizon.  It provides
 // structs that embed it with access to the App struct.
-//
-// Additionally, this type is a trigger for go-codegen and causes
-// the file at Action.tmpl to be instantiated for each struct that
-// embeds Action.
 type Action struct {
 	actions.Base
 	App *App

services/horizon/internal/docs/notes_for_developers.md

@@ -18,9 +18,8 @@ Compile and install Horizon as described in the [Horizon development guide](deve
 
 Horizon uses two Go tools you'll need to install:
 1. [go-bindata](github.com/kevinburke/go-bindata) is used to bundle test data
-2. [go-codegen](https://github.com/nullstyle/go-codegen) is used to generate some boilerplate code
 
-After the above are installed, run `go generate github.com/stellar/go/services/horizon/...`. This will look for any `.tmpl` files in the directory and use them to generate code when annotated structs are found in the package source.
+After the above are installed, run `go generate github.com/stellar/go/services/horizon/...`.
 
 ## <a name="scenarios"></a> Adding, rebuilding and using test scenarios