libstdc++: Remove unnecessary code from reduce implementation

Previously, this was necessary for efficient reductions. When I tried to
turn it into a missed-optimization PR, there was no issue (anymore). The
generic _S_static_permute implementation optimizes just fine (or better)
at this point.

The traits_impl.cc change is a simple drive-by fix.

libstdc++-v3/ChangeLog:

	* include/bits/simd_vec.h (_M_elements_shifted_to_front): Remove
	x86-specific implementation.
	* testsuite/std/simd/traits_impl.cc: Fix incorrect macro name.

Signed-off-by: Matthias Kretz <m.kretz@gsi.de>