Remove RESOLV_INJECT_CA_CERTIFICATE

The cppflags RESOLV_INJECT_CA_CERTIFICATE prevents test CA certificate
injection interface to be misused in production build. However, it will
also leads to resolv_integration_test fail while the resolver is built
for mainline release. Remove RESOLV_INJECT_CA_CERTIFICATE to get test
pass. The CA certificate injection interface is still protected by
permission check in DnsResolverService::setResolverConfiguration().

Bug: 147792436
Test: 1. build and install com.android.resolv from aml branch
      2. atest resolv_integration_test

Change-Id: Id03566dcaceab0f8c78fe705cb8f6f6c806082ca