Remove TxtStubLibraries.bp

The modules defined in TxtStubLibraries.bp are currently not in the
critical path of the from-text stub build, because the from-text vs
from-source switching is encapsulated within the stub library modules.
Therefore, remove the modules altogether.

Test: m nothing --build-from-text-stub
Bug: 287522618
Change-Id: I249edcdf596c8a82058e1bfb17da863c2b5dd7a7