storage: remove unneeded fields from Writer type
storage: remove unneeded fields from Writer type

Change-Id: If5e5d7b115c74b2320fcff1327ffac23b4cc8d7b
Reviewed-on: https://code-review.googlesource.com/3893
Reviewed-by: Michael McGreevy <e70fec140fa6773c6cfd45cb9b436bd6334f4c24@golang.org>

googleapis/google-cloud-go,michael-grosshans/google-cloud-go,googleapis/google-cloud-go,michael-grosshans/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go