Deprecate `group_static_libs` property

We now default to lld for platform builds, removing all need for this
property.  For more details, see "Effecient archive file handling" in
https://lld.llvm.org/NewLLD.html#key-concepts

Bug: 189475744
Test: Manual build succeeds
Change-Id: If1104d68b13de8c7afab35c1741a68f64394b448