[libc++][modules] Removes some validation quirks. (#88031)

Recent unrelated header cleanups caused these quirks to become obsolete.

Author: mordante

libcxx/utils/libcxx/test/modules.py

@@ -52,18 +52,13 @@
     "std::operator==",
 ]
 
-# Declared in the forward header since std::string uses std::allocator
-SkipDeclarations["string"] = ["std::allocator"]
 # TODO MODULES remove zombie names
 # https://libcxx.llvm.org/Status/Cxx20.html#note-p0619
 SkipDeclarations["memory"] = [
     "std::return_temporary_buffer",
     "std::get_temporary_buffer",
 ]
 
-# TODO MODULES this should be part of ios instead
-SkipDeclarations["streambuf"] = ["std::basic_ios"]
-
 # include/__type_traits/is_swappable.h
 SkipDeclarations["type_traits"] = [
     "std::swap",