fix redundant type warning

[fwojciec]

The most recent versions of gopls report explicit type definitions on individual slice/array/map items as a warning:

redundant type from array, slice, or map composite literal

I believe this is because of the inclusion of https://golang.org/cmd/gofmt/#hdr-The_simplify_command in the checks performed by the lsp.

The PR makes a small modification to codegen/generated!.gotpl template to prevent this warning from showing up in the code generated by gqlgen.

I have:

• Added tests covering the bug / feature (see testing)

It's effectively a code-formatting change and it doesn't affect any functionality.

• Updated any relevant documentation (see docs)

It's effectively a code-formatting change and it doesn't affect any functionality.

[andygrunwald]

To reproduce:
Running gofmt in simplify mode (gofmt -d -s .).

gofmt -d doesnt raise this piece.

[mtibben]

Seems reasonable