Remove remnant code of using indirect syscall on NetBSD

Summary:
The NetBSD version of internal routines no longer call
the indirect syscall interfaces, as these functions were
switched to lib calls.

Remove the remnant code complication that is no
longer needed after this change. Remove the variations
of internal_syscall, as they were NetBSD specific.

No functional change intended.

Reviewers: vitalybuka, joerg, javed.absar

Reviewed By: vitalybuka

Subscribers: kubamracek, fedor.sergeev, llvm-commits, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D52955

llvm-svn: 343988