Remove unused MockableSystemProperties from DnsManager constructor

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