hostmetricsreceiver: remove unused function

`gopsutil` recently added the capability to pass environment vars
through context. This is now done everywhere. This environment variable
setting function is no longer used or necessary. This PR removes it.

Signed-off-by: Braydon Kains <braydonk@google.com>

receiver/hostmetricsreceiver/factory.go

@@ -123,17 +123,12 @@ func createHostMetricsScraper(ctx context.Context, set receiver.CreateSettings,
 
 type environment interface {
 	Lookup(k string) (string, bool)
-	Set(k, v string) error
 }
 
 type osEnv struct{}
 
 var _ environment = (*osEnv)(nil)
 
-func (e *osEnv) Set(k, v string) error {
-	return os.Setenv(k, v)
-}
-
 func (e *osEnv) Lookup(k string) (string, bool) {
 	return os.LookupEnv(k)
 }