[gcc r15-1193] [libstdc++] drop workaround for clang<=7

[Alexandre Oliva via Gcc-cvs]

https://gcc.gnu.org/g:da57b4562c5ada8971b6684b6aad1c996e1ef9cc

commit r15-1193-gda57b4562c5ada8971b6684b6aad1c996e1ef9cc
Author: Alexandre Oliva <ol...@adacore.com>
Date:   Wed Jun 12 00:16:20 2024 -0300

    [libstdc++] drop workaround for clang<=7
    
    In response to a request in the review of the patch that introduced
    _GLIBCXX_CLANG, this patch removes from std/variant an obsolete
    workaround for clang 7-.
    
    
    for  libstdc++-v3/ChangeLog
    
            * include/std/variant: Drop obsolete workaround.

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