Use proper path in service file

Using /usr/bin/env is an antipattern.

Signed-off-by: Matt Spinler <spinler@us.ibm.com>
Change-Id: Ife47cd4eec90d280a329c8e7977cd1051e5d9c75
diff --git a/service_files/phosphor-button-handler.service b/service_files/phosphor-button-handler.service
index 79b5b05..de79594 100644
--- a/service_files/phosphor-button-handler.service
+++ b/service_files/phosphor-button-handler.service
@@ -7,8 +7,7 @@
 
 [Service]
 Restart=always
-ExecStart=/usr/bin/env button-handler
-SyslogIdentifier=button-handler
+ExecStart=/usr/bin/button-handler
 
 [Install]
 WantedBy=multi-user.target