linux: Remove INTERNAL_SYSCALL_DECL

With all Linux ABIs using the expected Linux kABI to indicate
syscalls errors, the INTERNAL_SYSCALL_DECL is an empty declaration
on all ports.

This patch removes the 'err' argument on INTERNAL_SYSCALL* macro
and remove the INTERNAL_SYSCALL_DECL usage.

Checked with a build against all affected ABIs.