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

cc/util.go

@@ -91,7 +91,6 @@ func flagsToBuilderFlags(in Flags) builderFlags {
 		systemIncludeFlags: strings.Join(in.SystemIncludeFlags, " "),
 
 		assemblerWithCpp: in.AssemblerWithCpp,
-		groupStaticLibs:  in.GroupStaticLibs,
 
 		proto:            in.proto,
 		protoC:           in.protoC,