fix unused variable error

The previous clean up left behind kSocketError which breaks in unittests:
shill/connection_health_checker_unittest.cc:62:11: error: unused variable 'kSocketError' [-Werror,-Wunused-const-variable]

BUG=chromium:385182
TEST=None

Change-Id: I0b6bd48cff80c6f9b3237ea680a85db9d8a44713
Reviewed-on: https://chromium-review.googlesource.com/205360
Reviewed-by: mukesh agrawal <quiche@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>

connection_health_checker_unittest.cc

diff --git a/connection_health_checker_unittest.cc b/connection_health_checker_unittest.cc
index 4b29421..e31aaa3 100644
--- a/connection_health_checker_unittest.cc
+++ b/connection_health_checker_unittest.cc
@@ -59,7 +59,6 @@
 const int kProxyFD = 100;
 const int16_t kProxyPortLocal = 5540;
 const int16_t kProxyPortRemote = 80;
-const int kSocketError = 3000;
 }  // namespace
 
 MATCHER_P(IsSameIPAddress, ip_addr, "") {