[libc++] Remove invalid C++20 code from a test.

During the review of D91986 it has been discovered the in C++11
deprecated `throw()` exception specification has been removed in
C++20. Removed the part of the test code using this feature.

GitOrigin-RevId: 99d5fad7a5cabac39d8a93485d1ad0ef4ba2a579
1 file changed