Remove unused imports (#1266)

Motivation:

We import NIOSSL unnecessarily in a few places.

Modifications:

Remove unused imports.

Result:

Fewer unnecessarily imports.

Sources/GRPC/GRPCChannel/GRPCChannel.swift

@@ -15,7 +15,6 @@
  */
 import NIOCore
 import NIOHTTP2
-import NIOSSL
 import SwiftProtobuf
 
 public protocol GRPCChannel {

Sources/GRPC/TLSVerificationHandler.swift

@@ -13,10 +13,8 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-import Foundation
 import Logging
 import NIOCore
-import NIOSSL
 import NIOTLS
 
 /// Application protocol identifiers for ALPN.