[libc][math] Fix -Wshadow warnings in cos.h (#196342)
cos() does `using namespace range_reduction_double_internal;` and
range_reduction_double_internal after 51e9430a0c767 contains
using LIBC_NAMESPACE::fputil::DoubleDouble;
using Float128 = LIBC_NAMESPACE::fputil::DyadicFloat<128>;
So the local using statements for DoubleDouble and Float128 shadowed
these. Just remove the local using statements.
No behavior change.
