Drop redundant GOOS build tags at start of file if the constraint is
already specified by the filename, e.g. foo_GOOS.go or
foo_GOOS_GOARCH.go, avoiding potential confusion in future.
cf. https://pkg.go.dev/cmd/go#hdr-Build_constraints
Signed-off-by: Daniel Swarbrick <daniel.swarbrick@gmail.com>