[patch] Remove duplicate libstdc++ tests

[Jonathan Wakely]

I noticed that testsuite/30_threads/shared_lock/modifiers/2.cc is
almost identical to 1.cc in the same directory, except for the diff
shown in the attached patch. That file was copied from
testsuite/30_threads/unique_lock/modifiers/2.cc which is almost
identical to 1.cc in that directory.

So I'm applying the attached patch to remove the unused variable name,
and deleting the duplicate tests.

Tested x86_64-linux, committed to trunk.

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