DnsManager does not use MockableSystemProperties to communicate
with netd to configure the dns server anymore. ConnectivityService
does not need to take it into DnsManager as a parameter. Thus,
clean up the unused code.
Bug: 33308258
Test: atest FrameworksNetTests
Change-Id: I2ac89944391f7d1fd5d67f26cf7f2b11a4a4cf84