hostmetricsreceiver: remove unused function (open-telemetry#29075)

**Description:**
`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.

**Link to tracking Issue:** open-telemetry#23055

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)
 }