delete context.Clear and dependency to gorilla/context

[KeiichiHirobe]

defer context.Clear(c.Request) does not make sense now because this library depends on github.com/gorilla/sessions v1.2.0.

So delete defer context.Clear(c.Request) and, go mod tidy to delete dependency to gorilla/context(UPDATE: #131 made a new dependency to github.com/wader/gormstore/v2@v2.0.0, which depends on gorilla context, so gorilla/context still exists in go.sum ).

If version is lower than v1.2.0, we need this code to avoid memory leak.
ref: gorilla/sessions#175

Resolve: #92