[nfc][libomptarget] Drop parameter to named_sync

[nfc][libomptarget] Drop parameter to named_sync

named_sync has one call site (in sync.cu) where it always passed L1_BARRIER.
Folding this into the call site and dropping the macro is a simplification.

amdgpu doesn't have ptx' bar.sync instruction. A correct implementation of
__kmpc_impl_named_sync in terms of shared memory is much easier if it can
assume that the barrier argument is this constant. Said implementation is left
for a second patch.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D88474