Cleanup go.tools.mod (#15329)

- Update reference to mmgotool in go.tools.mod
This would automatically be bumped while running i18n-extract.
- We also remove some other redundant lines for mmgotool that went in.
This has to be removed manually because go mod tidy would automatically scan the entire
codebase which is not possible with this file.

go.tools.mod

@@ -4,7 +4,7 @@ go 1.14
 
 require (
 	github.com/jstemmer/go-junit-report v0.9.1 // indirect
-	github.com/mattermost/mattermost-utilities/mmgotool v0.0.0-20200814100246-5fc14ed48292 // indirect
+	github.com/mattermost/mattermost-utilities/mmgotool v0.0.0-20200821090819-f7569391b4d3 // indirect
 	github.com/philhofer/fwd v1.0.0 // indirect
 	github.com/reflog/struct2interface v0.6.1 // indirect
 	github.com/tinylib/msgp v1.1.2 // indirect