Drop path property from cpp filegroup modules

The value of `path` here should be a no-op in Soong's cpp compile
action. Dropping this property helps bazel migration since Bazel's
filegroup rule does not have a path property.

Test: m libnetworkstatsfactorytestjni libservice-connectivity
Bug: 297405814

Change-Id: I7879224bf6deb12417d92e9a3fa52f58fccac999