go-helloworld: remove unused GO_WORKDIR

go-helloworld: remove unused GO_WORKDIR

The GO_WORKDIR is used only in go-mod.bbclass. As this recipe does
not inherit go-mod.bbclass, this variable is useless here.

This go-helloworld recipe was made to inherit go-mod.bbclass and build
in module-aware mode. However, it was found that we need to build go
recipes in GOPATH mode in order to support offline build. As a result,
this recipe was changed back to only inherit go.bbclass. But the GO_WORKDIR
setting was not cleaned up.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>