Remove XFAILs that are apparently for incorrect reasons in string tests

Author: ldionne

libcxx/test/libcxx/strings/basic.string/string.capacity/allocation_size.pass.cpp

@@ -8,11 +8,6 @@
 
 // <string>
 
-// This test demonstrates the smaller allocation sizes when the alignment
-// requirements of std::string are dropped from 16 to 8.
-//
-// XFAIL: using-built-library-before-llvm-19
-
 #include <algorithm>
 #include <cassert>
 #include <cstddef>

libcxx/test/std/strings/basic.string/string.capacity/max_size.pass.cpp

@@ -10,12 +10,6 @@
 
 // XFAIL: FROZEN-CXX03-HEADERS-FIXME
 
-// After changing the alignment of the allocated pointer from 16 to 8, the exception
-// thrown is no longer `bad_alloc` but instead length_error on systems using new
-// headers but a dylib that doesn't contain 04ce0ba.
-//
-// XFAIL: using-built-library-before-llvm-19
-
 // <string>
 
 // size_type max_size() const; // constexpr since C++20