Remove unused variables

From aosp/537809, variables that used to fetch realtime
stats are defined in NetworkStatsService. These varialbles
are filled by JNI in boot up stage in order to keep
definitions sync with native layer.

However, there is still a copy in TrafficStats.java, and this
copy cannot be filled in boot-up stage since it is in app
process. Besides, making a binder call to fetch these
constants from service is considered an overkill.

Thus, since there is no caller to these variables and callers
should use definitions in TrafficStats, remove these variables.

Test: atest FrameworksNetTests
Bug: 16229221
Change-Id: I6a48d4dbb1b824cfc6c4a47395b2a76aa28cf5c9