Print default val if all vals in axis match default val

To avoid verbosity, we currently dedupe keys in axis if its value
matches the value of //conditions:default. For axes where all values
might match the default value, we would effectively drop the common
value.

To fix this, we are now dropping the select statement and not the common
value.

Test: go test ./bp2build
Change-Id: Ic377b93ee2aba971753f6a5e7a62e15d1fcfa2bc