[libcxx][test] Remove redundant semiregular checks for CPOs

Some individual test files verify the CPO under test satisfies
`semiregular` concept.  This is redundant since it is already part of the test
in verifying whether the entity is indeed a CPO in
`libcxx/test/std/library/description/conventions/customization.point.object/cpo.compile.pass.cpp`.

Differential Revision: https://reviews.llvm.org/D116173