go.mod: Remove exclude directive

Remove the exlude directive that prevented us from pulling in a a buggy
version of grpc/grpc-go. We don't use this package anymore, so there's
no reason to keep this directive. Also, newer versions of grpc/grpc-go
has fixed the bug that's mentioned as the reason for the exclude.

Changelog: other

go.mod

 module gitlab.com/gitlab-org/gitaly/v14
 
-exclude (
-	// grpc-go version v1.34.0 and v1.35.0-dev have a bug that affects unix domain docket
-	// dialing. It should be avoided until upgraded to a newer fixed
-	// version. More details:
-	// https://github.com/grpc/grpc-go/issues/3990
-	github.com/grpc/grpc-go v1.34.0
-	github.com/grpc/grpc-go v1.35.0-dev
-)
-
 require (
 	github.com/beevik/ntp v0.3.0
 	github.com/cloudflare/tableflip v1.2.2