[committed 4/9] libstdc++: Remove redundant -std=gnu++17 options from concurrency tests

[Jonathan Wakely]

GCC defaults to -std=gnu++17 now anyway, and using it explicitly in the
dg-options directive prevents running these tests with different modes
such as -std=c++17 or -std=gnu++20.

Tested powerpc64le-linux. Committed to trunk.


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