Unecessary external dependencies are being downloaded.

[JeremyLoy]

Description

When declaring gin as a dependency using the latest master commit, unnecessary dependencies are declared in your go.mod/go.sum due to the examples/ directory containing executable go programs.

Those programs should be removed or moved to another Github repository.

Steps to reproduce

1. make a sample hello-world application using go modules and gin.
2. declare gin@master as a dependency using go get github.com/gin-gonic/gin@master
3. observe that unnecessary dependencies such as google.golang.org/grpc v1.15.0, google.golang.org/appengine etc are added to your own go.sum

[thinkerou]

#1569 merged!

[thinkerou]

ref: golang/go#28156 #1604

[xiaost]

I think https://github.com/gin-gonic/gin/blob/master/tools.go is one of the evils.
Many dependency management tools may introduce grpc by the import in tools.go

[thinkerou]

@xiaost tools.go have removed!